Both the Republican and Democratic parties have what are called Unpledged Delegates aka Superdelegates. They were developed by the Democratic Party in 1972 and then the Republican Party created their own version. Pledged Delegates for each State are dictated 100% by the will of the voters. Unpledged Delegates are actual individual people who get to vote in any way they like. They also can switch their vote at any time.
Each State has a certain number of Pledged Delegates based on the State's population and sometimes on how many from that party voted in the previous election. Some States are winner-takes-all, so the candidate who wins it gets all that State's Pledged Delegates. Some States are proportional, so the candidates get a percentage of the Pledged Delegates, based on the percentage of votes they get. Either way, the Pledged Delegate totals are based in one way or another on the way people in a State voted, and there is no negotiating. These Pledged Delegates are fixed. Each party has slightly different rules on these, but they are pretty much the same.
For the Democrats, there are 4,051 pledged delegates in the 2016 cycle. For the Republicans, there are 2,334. The difference doesn't matter. They just have two different systems that have zero effect on one another, and no interaction ever whatsoever.
The Republican and Democratic parties also have "Unpledged Delegates" (which is the official term; the unofficial slang term for them is "Superdelegates"). These are individuals who get one vote each to give to any candidate of their choosing. They have two super powers: 1) They can vote for anyone they want to no matter what, and they can cast that vote any time they want. 2) They can withdraw their vote at any time, recast it at any time, change it to someone else at any time. Those are some super powers!
The Republicans have 3 Superdelegates per State and Territory (I think): The State's Republican Party chair and of the State's Committee members. That makes 168 Republican Superdelegates (about 7% of their total Delegates).
The Democrats have 722 Unpledged Delegates (about 16% of their total Delegates). The 713 is made up of:
Okay, so you're surely wondering, WHY ARE THERE SUPERDELEGATES? There are two main reasons that stem from the same concern. But there's a good story to it:
In 1968, the Democratic Party's selection process for their candidate was a disaster. It's pretty fascinating, really. They had President Lyndon Johnson as the incumbent. And as 1968 started, Eugene McCarthy was the only Democrat to run against him, solely to bring attention to the anti-war movement. But the Vietnam War seemed to be winding down, so McCarthy's bid seemed just symbolic, and not a threat. But at the end of January, North Vietnam launched a huge surprise attack throughout Vietnam (80,000 troops attacking 100 towns and cities). The US was not ready for it.
McCarthy mobilized anti-war students to campaign door-to-door in New Hampshire (the first primary of the year (March 12)). McCarthy won 20 of the 24 delegates!! Four days later, Robert F. Kennedy announced his candidacy (sensing Johnson's vulnerability). And 2 weeks after that, President Johnson announced "I shall not seek, and I will not accept, the nomination of my party for another term as your President." And 4 weeks after that, Vice President Hubert Humphrey announced he was running.
The party was in disarray, and breaking into factions: 1) Unions and career politicians and their supporters (Hubert Humphrey), 2) Anti-war, mostly people under 30 (Eugene McCarthy), 3) Civil rights advocates (Robert Kennedy), 4) conservative Southerners (no one. They'd vote for Nixon and for a third party candidate, George Wallace (a segregationist)).
Only 15 States in 1968 had primaries. In April, McCarthy won Wisconsin, Pennsylvania, and Massachusetts. In May, Kennedy won Indiana and Nebraska; McCarthy won Oregon. Then on June 4, there were 3 primaries: McCarthy won New Jersey. Kennedy won South Dakota and California. California was a huge victory. Kennedy gave a great speech, and as he walked backstage, he was assassinated.
Talk about disarray going into the Convention in Chicago. Delegate totals were Humphrey 561 (all from States without primaries), Kennedy 393 (dead), and McCarthy 258. The Kennedy supporters were starting to support George McGovern. War protests took place in Chicago, and Mayor Daley cracked down on them hard with police force. The Convention itself was a publicity nightmare. Humphrey won on the first ballot. Although he had a lot of the support of the party's old guard, the youth movement was not impressed. Time magazine said "The old Democratic coalition was disintegrating, with untold numbers of blue-collar workers responding to Wallace's blandishments, Negroes threatening to sit out the election, liberals disaffected over the Vietnam War, the South lost. The war chest was almost empty, and the party's machinery, neglected by Lyndon Johnson, creaked in disrepair."
To put the Party back together McGovern chaired a committee to work on a new plan of that led to every State having a primary. The people's voice would be the primary source of all delegate assignments. Party leadership was reluctant to give 100% of that over power to the people, especially if States could have open primaries if they wanted to (open primaries mean that anyone can vote, not just Democrats). So superdelegates were developed: 1) To assure Party leadership that their wisdom would still be part of the process; 2) So that Party leadership could intercede if the open primary system led to an improper candidate or if the masses selected a candidate dangerous to the Party's survival.