Election Day is held on the first Tuesday in November in the United States. In each of the 50 states, and in the District of Columbia, there are political offices to fill, ballot questions to resolve, and every four years, the choice of whom to send to the White House.
The United States Constitution is filled with rights and protections for citizens. But as Georgetown University professor Mark Rom points out, voting is not necessarily one of them.
"One would think that the right to vote is just as much a central right of citizenship as the ability to speak freely, or to worship freely, to petition the government, and so forth. But, it is not," said Rom. "Voting requirements are typically established by the states. And, the states, more or less, can use their own standards."
The minimum voting age of 18 is set by U.S. federal law. But Rom says otherwise, there is very little uniformity
"There is a patchwork of 50 different states, with 50 different state laws about the times for voting, the processes for registering to vote, the places where you can vote, how you can vote by absentee ballot. All those details of the elections are established by state law, not by federal law," Rom added.
The absentee ballots that Rom refers to are used by people who are away from home on Election Day - in the military, for instance, or traveling, or away at school.
In recent years, something called "early voting" has also developed. Some two-thirds of the states now allow people to cast their ballots as much as a month or more before the official Election Day in November.
A number of states have now passed laws requiring voters to produce specific photo IDs at their polling place to prove their identities. Advocates of these laws say they are necessary to prevent fraud. Opponents say such fraud is too statistically small to be a problem, and charge the laws are meant suppress voting.
And there's also another way that a U.S. presidential election is really 50 separate contests - according to the Constitution, people voting for president are actually selecting 538 people called "electors" on a state-by-state basis. And it's these people who officially determine who becomes president.
"The most important number is 270," says George Mason University professor Dennis Johnson. "And that is the number of electors that will get you over the top. And, any combination of states that has 270, that is the magic number that you are really looking for."
Johnson says the Electoral College is why candidates spend most of their time in a handful of so-called “battleground” states -- the ones they believe will give them that winning total of 270 electoral votes.