I thought I was going to end up writing a lengthy post explaining who the Democratic Party's superdelegates are and why are they important for this election. This video, created by Obama Clock : Countdown to Barack Obama's Electoral Victory, is actually meticulously factual and unbiased.
It doesn't cover how the superdelegates came to be formed. For that you can go to the Wikipedia page, Superdelegates, where they outline the "raison d'etre" for their creation:

After the 1968 Democratic National Convention, the Democratic Party made changes in its delegate selection process, based on the work of the McGovern-Fraser Commission. The purpose of the changes was to make the composition of the convention less subject to control by party leaders and more responsive to the votes cast during the campaign for the nomination.

But some Democrats believed that these changes had unduly diminished the role of party leaders and elected officials, weakening the Democratic tickets of George McGovern and Jimmy Carter. In 1982, a commission chaired by former North Carolina Governor Jim Hunt created superdelegates. Under the original Hunt plan, superdelegates were 30% of all delegates, but when it was finally implemented in 1984, they were 14%. The number has steadily increased, and today they are approximately 20%.

The video though breaks down the math of how much a candidate needs to win in order to get the nomination in the Democratic Party. I will be coming back to that math in a subsequent post because it's part of the growing rift that's happening in the Democratic party.
In the meantime, I'll turn this page into a quick index of online resources about the subject:

• Democratic National Convention: Delegate Selection Rules[PDF documents].
• FactCheck.org : Who are the superdelegates and can they change their votes once they are committed?
• 2008 Democratic Convention Watch : They have a page with Superdelegate endorsements by candidates and the list of uncommitted superdelegates.
• Superdelgates.org: A wiki that is updated daily with the latest ups and downs of pledges, endorsements and elections results.

I just have to say that, as a progressive who has voted into office many candidates from the Democratic Party, I feel broadsided by these primary rules. The Democratic Party's definition of Democracy is just a tad hyper-republican and anti-democratic for my taste.