A delegate is a person who is generally empowered to represent a larger group. Every state has different amount of delegates. Delegates are either bound or unbound to a particular canidate. Unbound delegates havent chosen a particular canidate and even bound delegates can change their vote at the convention, if there is a brokered convention, (meaning no canidate was able to get the pre-requisite number of total delegates to win the nomination.)
The mass media says McCain has enough delegates to win the nomination of the Republican Party based on predictions, (exit polls) but some people don't think he has because there are still caucausus happening where delegates are still being picked. The mass media reports that all unbound delegates will be voting for McCain but that may not be neccessarily true. (To that effect, he may not even have enough delegates to actually be the winner, that will be announced at the convention, nearly 7 months away). From what I understand even bound delegates can change their vote, although some disagree. If someone knows more about this I would love to hear it.
The voting proccess is complicated, (on purpose, no doubt).