Voters are typically asked 20 questions on who they have voted for, demographic factors including age, gender, and race, and the reasons for the way they voted.
News organisations can start reporting results from the exit poll after the embargo period ends at 5pm ET on election day.
It can only forecast a winner in a race if all the polls have closed in the state, and even then it has to be a state where the margins of victory are sufficiently large.
How accurate are exit polls?
Election-watchers should be sceptical of the first exit poll results when they are released at 5pm ET.
These results will be adjusted several times over the course of the night because polling locations are still open. Those who have been surveyed earlier in the day tend not to be representative of the wider electorate.
For instance, those who cast their ballots earlier in the day tend to be older than the average voter. Republicans have an edge among those aged over 50 years old.
As the night goes on and more voters are added to the sample, the exit polls move closer to the final result.
Even then, the polls go through modelling that can dramatically alter their final conclusions.
The 2016 exit poll made headlines when it suggested that Donald Trump had won the support of the majority of white women.
In fact, when reviewed by the Pew Research Centre, it was 47 per cent of white women who supported Trump – cutting the initial figure by six percentage points.