Today, millions of Americans will head to the polls.

It's one of the biggest election days of the year, second only to November's presidential election.

Here's your really simple guide to Super Tuesday.

1. What's happening?

Americans of both parties will head to the polls to vote in presidential primaries — the process to decide who will contest the 2020 presidential election.

All of the focus will be on the Democrats, because US President Donald Trump will be the Republican nominee.

Republicans are technically holding a 2020 primary, but the winner will be Donald Trump. ( AP: Evan Vucci )

Americans in 14 states — Alabama, Arkansas, California, Colorado, Maine, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee, Texas, Utah, Vermont, and Virginia — will head to the polls.

The first polls will close at 11:00am AEDT. The last will close at 3:00pm AEDT. Results will come in until well into this evening.

Go deeper: We'll get you up to speed on the entire process Americans use to select their presidential candidates right here.

2. Why is it so important?

Because it's the day when the largest group of Americans will make a choice for the person they think is best to take on Mr Trump.

Winning on Super Tuesday awards a huge number of delegates as well. To become the Democratic nominee, a candidate needs 1,990 delegates.

On Super Tuesday, 1,357 delegates are up for grabs — the single biggest bounty in the entire primary process.

Success on Super Tuesday can literally make or break a campaign.

Once the dust has settled we'll have the clearest idea yet of who will be the person taking on Mr Trump in November.

What's a delegate?: They're like points, but also real people who will go to a convention in the middle of the year. We'll answer more of your delegate questions here.

3. What are the important bits to watch?

Pay attention to the number of delegates each candidate is winning. The more delegates they have, the better they're doing.

There are also two states you want to pay attention to — California and Texas. They're the most populous US states, and they also have the two biggest delegate prizes (415 for California and 228 for Texas).

Texas has been a Republican stronghold in the south for generations, but changing demographics mean it's slowly turning Democratic.

Who Texan Democrats vote for in the primary, and how many of them actually show up to vote, will give us a fascinating insight into one of the key states at the November election.

Michael Bloomberg has shaken up the race by spending millions on television and radio ads./ ( AP: John Locher )

Finally, everyone will be talking about billionaire Michael Bloomberg. It's the first chance anyone has had to vote for him, so it's worth seeing if spending hundreds of millions of dollars on television ads will pay off for him.

Want to know more?: Here's a long look at Michael Bloomberg, and how his money is upending this campaign.

4. Who is going to win?

There might be multiple winners. And multiple losers.

Until now only one state has voted at a time, so it's been very easy to say "Bernie Sanders won Nevada".

We may not have a good idea of who has "won" Super Tuesday on the day itself.

Postal votes, particularly in Texas, will be counted for days to come. And with its huge bounty of 228 delegates, who wins in Texas will be very important.

Get caught up: Here are the people still left in the race, and a little bit about each of them.

5. What happens afterwards?

More voting!

Despite how important Super Tuesday is, it still arrives very early in the primary process.

There will be more primaries and caucuses running all the way until June. It's likely we'll know who the nominee is before that though.

But, make sure you tune in next week. Another six states hold their own votes on what is sometimes called Super Tuesday II.

Grab your calendar: Here are all the key dates to pay attention to in US politics in 2020, including the date of the presidential election.