Gmail topped 900 million users in 2015. Not bad for a project whose code name came from a Dilbert cartoon. When it launched, no one believed it -- it debuted on April Fool's Day.

Partly because of Gmail's no-nonsense interface, people rarely take the trouble to learn its many features. After all, it's already so easy. But for those who take the time to learn its ins and outs, Gmail becomes a far more powerful tool. Here are 20 tips to help get you on your way to becoming a Gmail ninja.

1. Use the Gear to Enter a World of Settings

This is the single most important tool to navigating Gmail. If you're like the majority of people, you've probably never used it. Click the Gear -- located at the top right of the Gmail screen -- it and enter a brave new world.

2. Adjust the Display Density

The first thing you'll notice when you click the Gear are a number of choices under a heading called Display density. This adjusts the tightness with which the emails appear next to one another. It's helpful when working with different size screens. Select whichever option suits you best.

Display density changes how tightly emails appear together on the screen. Image Credit: eHow

3. Set the Number of Emails on the Page

You can also choose how many emails to display on a page. Click the Gear and then Settings > General. Under Maximum Page Size you can select the number of conversations (ie, emails) per page, from 10 to 100.

Remember that whenever you make a change in Settings, you must go down to the bottom of the page and select Save Change or your changes won't be kept.

4. Choose a Different Visual Theme

Choosing your Theme is one of the most dramatic visual changes you can make to Gmail. Go to the Gear and select Themes. You'll be greeted by an assortment of backgrounds. A colorful background gives you the feeling that you're working right from your desktop instead of within a browser. And you can choose one of your own pictures as your background as well.

If you want to go back to Gmail's default background, choose the one marked Light.

Gmail offers a generous selection of colorful backgrounds. Image Credit: Themes

5. Change the Font and Text Style

You can also choose your text style. Go to the Gear and choose Settings > General and you'll see Default text style. Here you can choose the font, style and color of your text.

6. Set the Conversation View

How do you fee about your threads? We don't mean clothes. Threads is a feature in many email programs that groups messages with their replies. You may or may not like this; perhaps you want to see each email displayed separately. If you don't want your email conversations nested within one another, you can turn that feature off. Head to the Gear and select Settings > General and then Conversation View. Choose the radio button next to Conversation view off to put an end to threads.

7. Set Your Inbox Style

You can also change your inbox style. Go to the Gear and choose Settings > Inbox. Select the drop-down menu beside Inbox type. It's set to default, which presents emails in the order they were received. But maybe you'd prefer another way. Maybe you'd like to see unread emails first, or starred emails, or even important emails at the top.

But you ask: How can Gmail know what's important? Excellent question! And that brings us to our very next tip.

8. Train Your Inbox

You can teach Gmail which messages are important to you. There's a small flag-like icon next to each email that turns yellow when clicked (see the screenshot for details). By clicking it you tell Gmail it's important. After a while, Gmail catches on and starts automatically identifying those emails as important. For instance, every email that comes in from your boss will be marked important. Conversely, you can deselect emails that are not important but which Gmail mistakenly identifies that way. Soon enough, you'll have Gmail doing the work of sorting your email for you.

The importance tab appears right next to the name of the sender in your email list. Image Credit: eHow

9. Organize Email Visually with Labels

Organizing your emails visually can help you identify important emails as you give your inbox a quick once over. Gmail's two main tools to do this are labels and stars. Let's start with labels, which also double as folders in Gmail. To create a label, you go to the top of the window and select the Move To icon, shaped like a folder.

The label icon is located in the top menu bar. Image Credit: eHow

When you click on it, you'll get a drop-down box. Select Create new. Give your new label a name and click Create.

Type in the name of your new label and click 'Create'. Image Credit: eHow

As we mentioned, labels are really just the name that Gmail uses for folders. They appear along the left-hand side of your inbox under the Compose button. You can make labeled emails pop more by assigning them a color. If you hover over a label name, you'll see a small arrow to the right. Click on it and a box will appear. At the top of the box you'll see Label color. Click on the arrow to the right of that and a label color selection box will appear. Simply click on the color you want and that will be the color of your label.

Choose the arrow to the right of the label name and the select label color at the top of the drop-down box. Image Credit: eHow

10. Organize Email with Stars, Too

Stars are another great way to make your emails stand out visually. Look to the right edge of the email list in your inbox, and you should see hollow stars. When you click on a star, it will turn yellow. Boom, you've now flagged the message. It can mean whatever you want; starring emails might tag the email as important, needing followup, or something else. It's up to you.

Click the star and it will turn yellow. Image Credit: eHow

11. Make Different Color Stars

Yellow is great, but you have many more colors to choose from as well, greatly expanding the organizational value of these little icons. Each time you click a star, it cycles through one of the available colors.

But here's a trick: You control which colors (and even shapes) are available. Click the Gear and select Settings > General. Scroll down to Stars. You can drag stars (and shapes) between the Not in use and In use sections, giving you all the power in determining what happens when you click stars back in the inbox.

Once you've selected what you want don't forget to click Save changes.

You aren't limited to only yellow when organizing visually using stars. Image Credit: eHow

Once you're back in your inbox, you can select the color of the star you want (or other icon) by clicking repeatedly on the star icon next to your email. It will successively run through your color options.

12. Create Email Filters

Creating filters is a great way to improve Gmail productivity. Essentially you're creating rules that automate Gmail's response to incoming emails. This saves you time in that you no longer have to deal with each email individually. The rules can be anything, from moving emails, to forwarding them, to sending a canned response.

First, select the email for which you want to create a filter. Then select More from the top menu bar and go down to Filter messages like these.

Once you've selected your email to filter, click 'More' and 'Filter messages like these'. Image Credit: eHow

In the example below, we've selected a marketing email from the Phoenix symphony. The first box that comes up has the From: line filled out. Click where it says Create filter with this search.

In this box, you want to select 'Create filter with this search'. Image Credit: eHow

Once you've finished, you'll be brought to another box where you can create the rule, or filter, that you want. In this case we're going to create two rules: skip the inbox and categorize these types of emails as a promotion. After you select your rules, click Create filter. That done, all emails from the marketing department of the Phoenix symphony will pass by the inbox and go directly to Promotions.

In our example, we've created two rules for our filter: Skip the inbox and put in the promotions category. Image Credit: eHow

You'll see in this box that you have many options to create a filter. You can star all emails coming from the sender, (in this case phoenixsympony.org) apply a label, mark them as read, or delete them, among other possibilities.

By creating filters you will introduce a flow to your inbox that keeps emails moving in the direction you want, adding mightily to your email efficiency.

13. Run a New Filter on Email Already in Your Inbox

When you create a filter, it won't have any effect on emails you've already received -- unless you select Also Apply filter to matching conversations which is a checkbox immediately to the right of the Create filter button.

'Also apply filter to matching conversations' ensures existing emails are filtered accordingly. Image Credit: eHow

14. "Unsend" an Email

You've probably sent at least one email you wish you could get back.

Wish you could click undo moments after you clicked Send? Fortunately, in Gmail you can do just that. Click the Gear then Settings > General. Click the checkbox to enable Undo Send and choose the amount of time you want to give yourself. Keep in mind that the longer you set the undo grace period, the longer it will take each email to actually get sent from your outbox, so 10 or 20 seconds are probably sufficient.

15. Master Keyboard Shortcuts

Keyboard shortcuts are a great tool to more quickly move through Gmail. You can use them to navigate around Gmail and also to help you sort your messages. The first thing you'll need to do is turn keyboard shortcuts on since their default position is disabled. You do this by clicking the Gear, then Settings > General. Go to Keyboard shortcuts and select the radio button beside Keyboard shortcuts on.

For a complete list of Gmail's keyboard shortcuts, visit here. You can also find a Gmail cheat sheet here.

16. Receive Other Email Accounts in Gmail

Most of us have more than one email account. And it's possible to add these accounts to Gmail, so you don't have to log into multiple accounts to check your email, as long as they're ordinary POP3 accounts.

You'll need to know the settings for the email accounts you plan to add. Once you have that information, go to the Gear, then Settings > Accounts and Import. Then select Add a POP3 mail account you own.

From here, just enter the inforation that Gmail asks for. Gmail will attempt to populate some of this automatically, but it may get it wrong. You're better off knowing this information ahead of time and entering it manually. You'll also notice a button next to Leave a copy of retrieved messages on the server. Select this if you check mail often for that account from other places. Once you have filled in the information, click Add account. You have now set up Gmail to receive emails from your other account.

17. Send Other email from Gmail Too

You'll also be asked if you also want to be able to send emails from this account. If you do select yes, you'll notice in the next box you'll be asked if you want to Treat as an alias. This is a personal choice that affects search settings more than anything. You can find out more details here. Essentially, if you select Treat as an alias, when you search "From: Me" Gmail will treat this new address also as "Me" -- meaning you -- and pull up all your emails from the new account you've added.

You'll also be asked to fill in your outgoing server information and password. When you click next, you may be asked for a confirmation code proving you indeed own that account, which will either be sent via email or texted to your phone. With that step completed, you're ready to go. You'll see the new email under Send Mail as: in Accounts and Import under settings.

When you next go to send an email you'll notice a drop-down box in the From: line of your email. It's giving you the choice to send an email from your original Gmail account or the new account you've added. You may want to be careful that you're sending from the address you want to be sending from. There is a setting in Accounts and import that lets you Reply from the same address the message was sent to. With this selected, you should avoid any mistakes if your goal is to always have your to: and from: the same.

18. Select Everything

Adding other email accounts can be a little complicated. So we're following it up with a simple tip that can save you a lot of aggravation. Perhaps you've noticed that you've amassed an ungodly number of promotional emails. It's time to kiss them goodbye and use the precious 15GB of space Gmail allots you for more important things.

You might already know that if you click the Select All icon in the far left corner of the top menu bar, only 100 emails (or however many emails you've set for a single page) are selected. If you have lots of emails, it will take a long time to delete them 100 at a time.

Selecting all emails using the icon in the top menu bar only selects emails on that page. Image Credit: eHow

It turns out that the solution is hiding in plain sight -- and you've probably never noticed it. When you select all, if there are more emails in this folder or label than fit on the current page, you always get the option to select all of the emails. Click the link in the yellow message to select everything.

By selecting everything, you select all the emails at once. Image Credit: eHow

Once you select of the emails, click Delete. Done and done!

19. Faster Search with Search Operators

Search operators are the keyboard shortcuts of the search world. Learning them now will save you a lot of sifting later. They'll help refine your search when you're looking for that email in a haystack.

Search operators in Gmail include From: which, when followed by a name, will automatically find all emails from that person. Similarly, there's To: which specifies a recipient. For instance, if you want to quickly find all your fan mail to comic book legend Stan Lee you would type To:Stan Lee and the list would appear in your inbox. You can also search by subject with Subject:Comic books. (Note that there can't be any space between the colon and the search term.) You can also search by labels (Label:) and sort emails by attachments (Has:attachment).

There are many other ways to search and Gmail provides a list of search operators here.

20. Explore the Goodies in Google Labs

Google engineers frequently add "experimental" stuff to make Gmail better. Google Labs is where you can try out some of these new features first.

Dive right in by clicking the Gear and selecting Settings > Labs. You've just entered the testing area. Here you'll find stuff like an authentication icon for verified senders (a tool to prevent spammers from mimicking messages to make them look like they come from a trusted website). To try it out, click Enable.