On Election Day (November 7) U.S. citizens over the age of 18, who are registered voters, vote for their favorite candidates. This is known as the popular vote.
But the candidate, who wins the popular vote, doesn't necessarily win the election. The popular vote is used to determine the electoral vote of the Electoral College, which is made up of 538 people, who each get one electoral vote. The Electoral College is a group of people chosen within each state to elect the President and the Vice President.
Each state has a different number of electors depending on how many people live in that state. (California, a state with a big population, gets 54 electors, while Montana, a state with a small population, gets only 3.) See, how many each state has here!
Finally, the candidate, who gets the most electoral votes (270 or more), wins the election and becomes the next President of the United States.