Only two parties dominate the American political system, so the president always belongs to one of them.
The Republicans are the conservative political party in the United States and their candidate in this year’s election is President Donald Trump, who hopes to secure another four years in power.
The Democrats are the liberal political party in the US and their candidate is Joe Biden, an experienced politician best-known for serving as Barack Obama’s vice president for eight years.
Both men are in their 70s – Mr Trump would be 74 years old at the start of his second term, while at 78, Mr Biden would be the oldest first-term president in history.
Each state gets a certain number of electoral college votes partly based on its population and there are a total of 538 up for grabs, so the winner is the candidate that wins 270 or more.
This means voters decide state-level contests rather than the national one, which is why it’s possible for a candidate to win the most votes nationally – like Hillary Clinton did in 2016 – but still be defeated by the electoral college.
All but two states have a winner-takes-all rule, so whichever candidate wins the highest number of votes is awarded all of the state’s electoral college votes.
Most states lean heavily towards one party or the other, so the focus is usually on a dozen or so states where either of them could win. These are known as the battleground states.
if you’re a US citizen and you’re 18 or over, you should be eligible to vote in the presidential election, which takes place every four years.
However, lots of states have passed laws requiring voters to show identification documents to prove who they are before they can vote.
These laws are often put into place by Republicans who say they’re needed to guard against voter fraud. But Democrats accuse them of using this as a form of voter suppression as it is often poorer, minority voters who are unable to provide ID like a driving licence.
How people vote is a contentious issue this year because of the coronavirus pandemic. Some politicians are calling for wider use of postal ballots, but President Trump has said – with very little evidence – that this could result in more voter fraud.