The delegates to the Democratic and Republican parties' national conventions officially choose the nominees for the presidency.
So anyone who wants to run for the presidency representing either party has to try to win the support of as many as possible - and the key is to do well in the state primaries and caucuses held between January and June.
A big vote in favour of candidate X, in a given state, usually mandates a certain proportion of that state's delegates to vote for candidate X at the convention.
Do all states have the same number of delegates?
No. There is a huge variation. The most populous states - California, Texas and New York - have many times more delegates than the smallest states.
Do both parties have the same number of delegates?
No. The Democratic Party convention will have almost twice as many delegates as the Republican Party convention in 2008.
But in both cases a candidate only needs a simple majority (50% +1) of delegate votes to win the nomination - that's currently 2,118 in the case of the Democrats, and 1,191 in the case of the Republicans.
The goal of all candidates is to win the support of as many delegates as possible, as early as possible in the primary season.
Even before a candidate secures the winning number of delegates, he or she may notch up an effectively unassailable lead.
Do all delegates have to vote at the convention as directed by the result of the primaries and caucuses?
No. Both parties have a certain number of elected delegates (also known as pledged delegates), whose vote is determined by the result of the primary or caucus in their state, but they also have a certain number of unelected/unpledged delegates (known as super-delegates in the Democratic Party).
These delegates are free to choose which candidate to support. Many of them hold elected office, but they do not owe their place at the convention to a primary election or caucus.
How crucial are the unelected delegates?
In both parties they form a minority within the ranks of delegates at the convention - about 20% in the Democratic case, and between 5% and 20% of Republican delegates (the Republicans do not provide an official breakdown, and different experts give different figures).
They are mostly high-ranking party officials, members of Congress and state governors.
In most years, candidates do not have to worry too much about wooing unelected delegates. But if the race gets very close - as it is in 2008 on the Democratic side - they cannot be ignored.
Is the number of pledged delegates a candidate wins in a primary or caucus always proportionate to the number of votes he or she receives?
No, not always. The rules vary from state to state and from party to party.
In some states the Republicans operate a winner-takes-all system, where the candidate who wins most support state-wide gets all the delegates.
In others, the winner-takes-all principle operates at the level of congressional districts: the candidate who does best in a district wins all the delegates available in that district.
The Republicans also use a proportional system in some states.
The Democrats always use some form of proportional system, but even then a candidate's share of the vote in a state and his or her share of the delegates can turn out to be quite different.
For example, when delegates are awarded on the basis of results in individual congressional districts, the rules do not guarantee strict proportionality.
Under party rules, it's possible for one candidate to beat the other soundly in a district with an even number of delegates, but for the delegates to be split between them equally nonetheless.
Meanwhile, in a district with an odd number of delegates, even a narrow win gives the winner an extra delegate.
Is it always clear who has won a primary?
It can be confusing when one candidate wins the most votes and another wins the most delegates.
In 2008, Hillary Clinton won more votes than Barack Obama in Nevada's and Texas's Democratic contests - but he won more delegates, according to AP's projections.
Are delegates awarded immediately after the primary or caucus?
After a primary, which takes the form of a state-wide ballot, delegates are usually awarded quickly.
Caucuses are a different matter. The candidates and the media focus only on the first stage of the caucus, when voters at precinct level choose delegates to send to the county caucuses.
But the caucus process often goes through several stages, ending only weeks later, at a state convention where delegates are chosen to send to the national party convention.
This does not stop experts projecting the final allocation of delegates from the results of the precinct caucuses, though they often come up with very different results.
When do unelected/unpledged delegates declare their support for a candidate?
They can do this any time they like. They can also change their mind before the convention.
How tightly bound are elected delegates to a given candidate?
It varies from state to state. In some cases they are not really bound at all.
In others they may be bound to support a given candidate in the first ballot held at the convention, and then be free to make their own choice.
Or they may be bound to support the candidate through two or three, rounds of voting, or even all the way to the final vote of the convention.
If no candidate accumulates a winning number of delegates before the convention, then what?
A convention that begins without a clear winner is referred to as a brokered, or contested convention.
If no winner emerges from the early ballots, the rivals may have to negotiate.
If candidate X offers candidate Y the Vice-Presidency, say, candidate Y's supporters may then help candidate X defeat candidate Z.
Could the numbers of delegates at the convention change?
The number of Democratic super-delegates changes regularly, as politicians leave office, or die, and are replaced by others.
The party also disqualified delegates from Florida and Michigan, when the states broke party rules by holding their primaries too early.
However, in June 2008, the Florida and Michigan delegates were re-instated - though only with half-votes. Finally, on the eve of the convention, the decision was taken to allow them full votes after all.