Prior to the race to the White House, which begins after the parties' national conventions in the summer, the Republicans must choose their preferred candidate to run against incumbent president and Democrat party leader Barack Obama. This is done through a series of state caucuses and primaries, which began in Iowa in early January and end in Utah in late June.
A caucus is essentially a meeting of political activists. Candidates are discussed and invited to speak about their policies, affiliations and beliefs.
For example, this year Republicans at each of the 1,784 precincts in Iowa, where the first caucuses took place on 3 January, met up, held discussions and placed their votes in a hat. Although the Democrats did not convene in Iowa this year because Obama is the incumbent president who will seek re-election, their selection process entails dividing themselves into groups according to the candidate they support and trying to swing undecided voters to back their candidate.
After the caucus meetings, a delegate, who has already been appointed, will attend their respective party's summer convention and vote for their state's chosen candidate. This summer the Republican National Convention will take place in Florida during the week of 27 August in Tampa, Florida.
Obama will be anointed as Democrat Party's chosen candidate the following week in Charlotte, North Carolina.
The caucus system, which is only used in 12 of the 50 states, has come in for criticism for being a less transparent and democratic selection process, because those with stronger voices tend to dominate meetings.
In contrast to the time-consuming process of holding a caucus, a primary is a more straightforward selection procedure, in which citizens go to polling stations to cast their ballots and the winner is the person with the most votes using the first-past-the-post system.
The primaries reach a climax on 6 March, when 10 states go to the polls on the same day in what has been dubbed as "Super Tuesday".
After the primary season, pre-appointed delegates also attend the summer party conventions to vote for their state party's preferred candidate.