I'll keep this real brief. To keep it real simple, the idea of a primary is that delegates from the state vote for a party candidate based on the candidate's popularity by votes. However, in the Republican party this is not the case, as states can choose to use a "winner-take-all" format, which many do. The Democratic party differs in that a large number of delegates are "unpledged", or to use the media's favorite term: "superdelegates". The fact that these delegates are unpledged means they can vote to nominate whoever they want to, regardless of citizens' votes. So, the potential canditates end up campaigning the delegates instead of the actual voters, especially in a race as close as this one.

Superdelegates to the 2008 Democratic National Convention include all Democratic members of the United States Congress, Democratic governors, various additional elected officials, members of the Democratic National Committee, as well as "all former Democratic Presidents, all former Democratic Vice Presidents, all former Democratic Leaders of the U.S. Senate, all former Democratic Speakers of the U.S. House of Representatives and Democratic Minority Leaders, as applicable, and all former Chairs of the Democratic National Committee." (Delegate Selection Rules for the 2008 Democratic National Convention -- link on right)

The 2008 Democratic convention will have (currently) 796 "superdelegates" (this can change right up until the convention). Delegates earned from caucuses and primaries total 3253, for a grand total of 4049. To earn the nomination, a candidate must obtain a majority, namely, 2025. The superdelegates play such a huge role, because when a candidate wins a state, they usually only gain a one or two delegate advantage, since the primary delegates are divided by percentage. So winning over one superdelegate is on par with winning a whole state.

Quick hypothetical example: A state has 20 pledged delegates. Candidate "A" "wins" the state with 50% of the vote. A recieves 10 delegates (50% of 20). "B" came in second with 45% of the vote. B recieves 9 delegates. So, by winning the state, A only gained one more vote than B. Likewise, winning over one superdelegate would also gain one vote for A.