Democratic Party

The Democratic Party uses pledged delegates and superdelegates. A candidate for the Democratic nominee must win a majority of combined delegate votes at the Democratic National Convention.

Pledged delegates are elected or chosen at the state or local level, with the understanding that they will support a particular candidate at the convention. Pledged delegates are, however, not actually bound to vote for that candidate, thus the candidates are allowed to periodically review the list of delegates and eliminate any of those they feel would not be supportive. Currently there are 3,253 pledged delegates.

Of the 4,047 total Democratic delegates, 794 are superdelegates, which are usually Democratic members of Congress, governors, former Presidents, and other party leaders. They are not required to indicate preference for a candidate.

The Democratic Party uses a proportional representation to determine how many delegates each candidate is awarded in each state. For example, a candidate who wins 40% of a state's vote in the primary election will win 40% of that state's delegates. However, a candidate must win at least 15% of the primary vote in order to receive any delegates. There is no process to win superdelegates, since they can vote for whomever they please. A candidate needs to win a simple majority of total delegates to earn the Democratic nomination.

Republican Party

The Republican Party utilizes a similar system with slightly different terminology, employing pledged and unpledged delegates. Of the total 2,380 Republican delegates (2,286 in 2012), 1,719 are pledged delegates, who as with the Democratic Party, are elected at the state or local level. To become the Republican Party nominee, the candidate must win a simple majority of 1,191 of the 2,380 total delegates at the Republican National Convention.

A majority of the unpledged delegates are elected much like the pledged delegates, and are likely to be committed to a specific candidate. Many of the other unpledged delegates automatically claim the delegate status either by virtue of their position as a party chair or national party committee person. This group is known as unpledged RNC member delegates.

The process by which delegates are awarded to a candidate will vary from state to state. Many states use a winner-take-all system, where popular vote determines the winning candidate for that state. However, beginning in 2012 many states now use proportional representation. While the Republican National Committee does not require a 15% minimum threshold, individual state parties may impart such a threshold.

The unpledged RNC member delegates are free to vote for any candidate and are not bound by the electoral votes of their state. The majority of the unpledged delegates (those who are elected or chosen) are technically free to vote for any candidate. However, they are likely to be committed to one specifically.

https://en.wikipedia.org/wiki/Delegate