When primary or caucus voters make their choice, they are actually voting for convention delegates pledged to that candidate. The contender who has the most delegate votes at their party's presidential convention wins the nomination. 

When the 2012 Republican National Convention convenes in Tampa, Florida, on August 27, a total of 2,286 delegates will be there. It will take 1,144 delegates for a candidate to be the Republican nominee on the November election ballot. 

The 2012 Democratic National Convention opens September 3 in Charlotte, North Carolina, where there will be 5,554 delegates. While it will take 2,778 delegates to nominate President Barack Obama for a second term, he is running unopposed within his own party. 

The two parties have different formulas for apportioning and committing delegates to presidential candidates. The Democratic Party's process is explained by Democratic National Committee press secretary Melanie Roussell.

"The state [Democratic] party either has a government-run primary, a party-run primary, or a caucus to determine how many delegates will be allocated from [to] that state, for a particular candidate at that convention," she said. 

Roussell says that in all states, delegates are awarded to candidates proportionally. If someone gets 35 percent of the vote in a state's primary or caucus, that candidate will get 35 percent of the state’s delegates. 

Those delegates are pledged to vote for that candidate's nomination at the party convention. 

But not all of the 5,554 delegates at the Democratic Convention are locked in to specific candidates. There are 727 delegates, sometimes called "super-delegates," who are state and national Democratic officials such as governors, senators, and House members. These 727 delegates can support any candidate they choose.

In the Republican Party, some states award delegates on a proportional basis as do the Democrats. But other states have a "winner-take-all" basis. It's tied to the calendar, says Republican National Committee communications director Sean Spicer.

"So what we did is that we came up with a system that basically said 'If you [a particular state] go [hold a primary or caucus] prior to, or within the month of March - any time before April 1 - you must be a proportional state," he said. "That ensured that you couldn't just rack up a couple quick wins, and run away with the nomination."

Delegates are pledged to specific candidates on the first roll call vote at the convention. But if one candidate does not lock in the nomination on the first vote, a convention may become "brokered," where factions both compete and combine to produce a presidential nominee. But, this has not happened since 1952.