In most U.S. elections, the candidate with the most votes wins. However, the President and Vice President are not elected directly by the popular vote. Instead, they are chosen through a process called the Electoral College.
Each state is assigned a certain number of "electors" based on its total representation in Congress (House members + Senators). There are 538 electors in total. When you vote for President, you are actually voting for a slate of electors who have pledged to support that candidate.
In 48 states and D.C., the candidate who wins the popular vote in that state receives *all* of that state's electoral votes. This is why candidates focus heavily on "swing states" where the race is close, rather than just trying to get as many individual votes as possible nationwide.
To win the presidency, a candidate must receive at least 270 electoral votes—a simple majority of the 538 available.
Because of the state-by-state allocation, it is possible for a candidate to win the nationwide popular vote but lose the Electoral College (and therefore the presidency). This has happened five times in U.S. history.