Introduced at the company’s annual Worldwide Developers Conference, Apple’s iCloud sync service has the potential to help millions of users keep their information ubiquitous among all their devices.

But what exactly is iCloud? How does it compare to MobileMe? How much will it cost? We answer these questions—and many more—below, split by section. Want to skip to a specific section? We answer questions on general information and storage, document and data sync, Photo Stream, purchase management and iTunes Match, location awareness, mobile backup, and troubleshooting iCloud problems.

General iCloud questions

What is iCloud?

In short, iCloud is a catchall phrase that covers Apple’s entire suite of wireless sync and backup services, which aim to keep your devices—both iOS, and desktop computers running OS X Lion, Windows Vista, or Windows 7—on the same page, no matter which one you’re using at any given moment.

Why is Apple doing this?

When Steve Jobs spoke about iCloud, he said that Apple was going to demote the computer to be “just another device.” So, rather than your Mac being the digital hub for your media and personal information, that job would be taken over by online services—specifically, iCloud. Given that now that many of us have not only multiple computers but also one or more mobile computing devices such as the iPhone, iPod touch, and iPad, this makes a lot of sense. Coordinating all your information between these devices has become a chore—particularly when you attempt to do it all from a single computer. The promise of iCloud is that syncing media and data will “just work.” Just enter your Apple ID on your various devices and iCloud will make sure that all those devices have the most up-to-date content on them.

Of course, there’s also the bigger picture: By creating a system where all your computing devices communicate seamlessly and let you access your media on demand, Apple is making it even more appealing to stay in the Apple ecosystem and to buy even more Apple devices.

Isn’t that what MobileMe does?

Yes and no. iCloud will offer some of the same features as MobileMe, but not all, and it will add new features, as well.

What does this mean for MobileMe?

MobileMe will be going the way of the dodo. More on that in a bit.

What does iCloud offer?

As with MobileMe, iCloud can sync your contacts, calendars, email, and Safari bookmarks between iOS devices and computers. But it will also offer document storage, photo storage, purchase management, location awareness, and mobile backup for iOS devices.

What will this cost?

All base iCloud accounts (which come with full sync features, backup, and 5GB of storage) are free. If you want to upgrade your storage or pay for Apple’s iTunes Match feature, you can pay a yearly fee.

Do I get an email address? Does it stay @me.com or change to something else?

You do! When you sign up for a new iCloud account, you’ll get to pick an email address that looks like this: user@me.com. If you already have an Apple ID and are upgrading to iCloud, you can choose whether or not to sign up for a me.com address; if you have a MobileMe, .Mac, or iTools account, you’ll be able to keep your current @me.com or @mac.com address when converting to iCloud.

Will it work with all my devices?

To use iCloud, you’ll need your devices to be running iOS 5 (if they’re mobile), OS X Lion (if they’re Macs), or Windows Vista or Windows 7 (if they’re PCs). Unfortunately, this means that Snow Leopard and iOS 4 users are—for the time being, if not permanently—left out in the cold.

How much will I be able to store on iCloud?

5GB of data with a free iCloud account. That storage space covers mail, documents, and mobile backup data. Your purchased music, apps, books, and Photo Stream photos (more on that below—won’t count against your 5GB limit.

What if I need more storage? Will there be a way to pay for more?

Yup. Apple offers several tiers of storage, priced by the year. You can upgrade to 15GB for $20/year, 25GB for $40/year, or 55GB for $100/year. If you change your mind, Apple also allows you to cancel at any time and receive a prorated refund (you’ll only be charged for the portion of the year you used the extra storage for, rather than the full amount), or select a plan to downgrade to at the end of your billing year.

You can purchase more storage from your Mac, PC, or on your iOS device. On your Mac, open the System Preferences app and click on the iCloud button. From there, click Manage, then Buy More Storage. On your PC, you can find similar preferences in the control panel, under Network and Internet. If you’re purchasing storage on your iOS device, tap the Settings app, then iCloud -> Storage & Backup -> Buy More Storage.

How does iCloud compare with Dropbox and SugarSync?

Unlike Dropbox and SugarSync, which are designed to let you sync any file or folder on your computer through their services, iCloud is focused on integration with apps: Any iCloud-enabled app will sync documents automatically to the service’s server. As such, you won’t be able to sync individual files from apps that don’t offer iCloud support.

Will parts of iCloud be accessible with a browser?

Yep. Just like MobileMe, you can see your Mail, Contacts, Calendars, and access Find My iPhone through your browser by going to icloud.com. In addition, you can access your iWork for iOS documents, and upload iWork documents from your Mac to iCloud so that you can work on them in your apps.

When will iCloud be up and running?

All iCloud features are available right now if you’re running iOS 5 on your mobile devices and Lion on your Mac. iTunes Match is available for those running iTunes 10.5.1 or later on OS X Lion or Snow Leopard.

Document and data sync

Will iCloud store and back up any file I want it to?

Not quite. iCloud will sync documents created with Apple’s apps (Pages, Numbers, and Keynote, for example) as well as any third-party apps that are adapted to take advantage of the service. Presumably, iOS and Mac developers will eventually get on the iCloud bandwagon, but there will be a transition period where some apps will sync with iCloud and others won’t. That means users will need to be careful not to assume that all their files are automatically saved to iCloud, because only some apps will do the job.

If you have iCloud’s mobile backup feature enabled, the service will store your app’s data—its documents, game saves, etc—but will not sync it to other computers or devices.

Will I be able to use iCloud to sync settings and other data between my Macs?

Other than the data mentioned—photos, media, mail, contacts, and calendars—not at this time. But it’s entirely likely that iCloud’s purview may expand to application data on the Mac: It may only be a matter of time before you’ll be able to sync, say, your application and system settings, and perhaps even your login sessions, between your Macs.

Can I choose what syncs and what doesn’t, per device?

Yep. You can selectively enable contacts, calendars, reminders, bookmarks, notes, Photo Stream, Find My iPad, and iCloud backup on and off individually on each device.

Can I sync with more than one iCloud account on my device?

You can, but only one iCloud account can be the primary account (associated with your Photo Stream, Find My iPhone, Backups, and document sync). Secondary (and tertiary) accounts can only sync mail, contacts, calendars, reminders, bookmarks, and notes.

What happens to data when you delete a game and reinstall it? Is there a way of restoring game saves?

If your game has iCloud support, your game saves can be synced, so you can pause on an iPad and pick up on an iPhone.

Photo Stream

What about my photos?

iCloud includes a feature called Photo Stream, which auto-imports any new pictures taken on an iOS device (or added to iPhoto/Aperture on your Mac) and stores them for 30 days. You can view those photos on (and in some cases, download them to) your other iOS devices, computers, and even your Apple TV. Because of size concerns, photo syncing is limited to 1000 pictures on iOS devices; you’ll see your full stream of images on any of your supported desktop computers. (Though you can’t run iPhoto on a Windows PC, Apple says Photo Stream can still work: you’ll simply choose a folder on your hard drive that you want to use as your “photo library”.)

Only 1000 images? What about all my other pictures?

While it’s true that iCloud only stores your most-recent images, that doesn’t mean you’ll lose older pictures. Any pictures sent to iPhoto (or, on a Windows PC, to its designated images folder) will be permanently stored on your computer.

Can I delete an image from my Photo Stream?

Unfortunately, no. You can reset your entire Photo Stream from icloud.com, but there’s no support for individually deleting a picture at this time.

Will using iCloud’s photostream on the Mac require me to upgrade to iPhoto ’11? Windows users don’t need an extra expenditure to use it.

Yes, you’ll need to upgrade to the latest version of iPhoto for $15, or own Aperture 3 if that’s your program of choice.

Purchase management and iTunes Match

What does iCloud do with my purchased iTunes content?

Using your free iCloud account, you can access a complete record of all your purchases from iTunes. Choose to download new music, apps, and books automatically; and redownload anything for free. If you’ve purchased tracks from the iTunes Store in the old 128-kbps protected AAC format and re-download them, they’ll be delivered in that same format—they won’t be offered to you as unprotected 256-kbps AAC files (for that, you’ll have to pay 30 cents a track to upgrade them through iTunes). If tracks that you’ve purchased are no longer available from the iTunes Store, you won’t be able to re-download them.

For a fee ($25/year), you can also enable iTunes Match, which allows you to stream and download any content you’ve bought from the iTunes Store onto your devices. You’ll also be able to upload any music you own that you didn’t originally purchase from iTunes (see the next question for more information).

What about music that isn’t from the iTunes Store (stuff I ripped from CDs myself, say)?

In addition to letting you stream and download any track from iTunes you own, iTunes Match will scan your iTunes library and match (if possible) any of your songs with ones from the iTunes Store, which can then be streamed and downloaded to any one of your iCloud-supported devices.

Even better, once iTunes Match has “matched” those tracks, it replaces low-bit rate versions with 256-kbps, DRM-free AAC versions. For songs that iTunes can’t find in its 19 million song library, you’ll be able to store up to 25,000 additional manually-uploaded tracks. iTunes Match was supposed to launch in late October, but Apple missed the release date; the company has no projected date for the official launch.

If I don’t renew iTunes Match after a year, do I lose those 256-kbps AAC versions of my ripped tracks?

No. You’ll keep any songs you’ve upgraded. If you unsubscribe, you’ll just lose access to streaming and downloading those tracks (and others) to your devices, and, unlike your iTunes-purchased content, you won’t be able to re-download them from the iTunes Music Store.

Can I stream my iTunes-purchased music directly from the Music app or iTunes on my computer, or do I have to first download them to a device?

If you have iTunes Match, yes; if you don’t, you can listen to a preview of your purchased music within the iTunes Store to help you identify a track before downloading it, but you won’t be able to access songs directly from within your Music app or iTunes.

I have a few albums I copied from a friend. How will iTunes Match know which files are legal for me to upload?

The specifics of how iCloud does or doesn’t deal with pirated music will likely be a secret. We’d guess that if you try to match tracks purchased from someone else’s iTunes account, you’ll be told that you don’t have the right to use them. Those tracks are watermarked with another Apple ID and therefore easy to identify. As for tracks that have been downloaded from the Internet, it’s possible that Apple could look for identifying characteristics that indicate the music came from a dodgy source. For example, tags, watermarks, and sonic footprints. On the other hand, it’s possible that part of the $25 you pay each year buys you a measure of amnesty from the labels.

What about other iTunes Store content, like movies and podcasts?

At this point, due to licensing restrictions, you can only re-download music, books, apps, and TV shows in the U.S.; other countries may not even have those options. We hope to see Apple extending this feature to other types of content (Movies, iTunes U) at some point, but we’ll have to wait until deals are worked out.

Most of my music is on a non-iOS iPod. How does the iPod fit into the scheme of songs being pushed to all devices?

iCloud requires an Internet connection to do its job, which explains why it’s limited to iOS devices—which can connect to Wi-Fi networks and, in most cases, 3G data networks as well. We can’t see how an iPod nano or classic could ever fit into the iCloud ecosystem.