Both the Republican and Democratic parties have delegates to their national conventions who aren’t bound to the results of the primary system. The Democrats call theirs superdelegates and there are 796 of them. Democratic superdelegates are elected officeholders and party officials.
The Republicans call them unpledged delegates and there are 463. 123 of these are Republican National Committee members, the remainder are elected in the primaries but aren’t pledged to a particular candidate.
There’s a pretty fair write-up of the superdelegate system here.
When you realize that the number of delegate votes needed for either Hillary Clinton or Barack Obama to secure his or her party’s nomination is 2,025 and the superdelegates are currently breaking almost 2:1 for Clinton, it’s pretty darned likely that, if the primaries continue neck and neck between Sens. Clinton and Obama, the superdelegates may be instrumental in deciding who’ll be the party’s nominee.