This great explanation of the Superdelegates was put together by commenter Ceabaird
What are they? Who are they? And what power do they have?
Basically, what we are doing right now is deciding who to nominate as the Democratic Presidential Candidate for the 2008 election. We are electing delegates to the national Democratic Convention, and how that happens is pretty confusing. Delegates are chosen from within the party for each state, territory or region to attend the convention and then select the eventual winner. For the most part, these people are drawn from each congressional district, although some are not. These others are chosen from the state (or region) at large, and include party leaders, state governors, and other "luminaries." From the 2008 Delegate Selection Rules for the Democratic National Convention:
...75% of each state's base delegation shall be elected at the congressional district level or smaller. 25% of each state's delegation shall be elected at large, the district-level and at-large delegates, respectively." States with more than one congressional district will certify pledged party leader and elected official (PLEO) delegates at 15% of the base delegation, before selecting the at-large delegates. These "at large members" for each state, must reside there, and include the following groups: members of the DNC, the Democratic President and VP (if applicable), all Democratic members of the US Senate and House, the Democratic Governor (if app.), and all former "...Democratic Presidents, VPs, Leaders of the US Senate, House Speakers, Minority Leaders (if app.), and all former Chairs of the DNC.
So, the 25% "at-large" delegates include the above special members, and the remainder is selected after the "unpledged add-on delegates," and then the PLEO delegates are chosen, to fill in the remainder of slots available. These "unpledged add-on delegates" are not allowed to have alternates.
The delegates earned through caucuses and elections are the "pledged delegates" (pledged to a presidential candidate based on the % of the primary or caucus voting results), while the ones selected due to membership in the DNC, members of congress, former presidents and others who have held or hold important political offices are the "Superdelegates." Super Delegates are recognized as part of their state's delegation, unless they have "...publicly expressed support, or endorsed, a presidential candidate of another political party." This is why Leiberman is gone.
Instituted after the fractured conventions in the 60's and 70's, this is primarily a way for the Democratic Party leadership to maintain a level of control over the primary, and therefore the selection of presidential candidates. Over 700 (790+) delegates are designated as "Superdelegates", or delegates separate from the general delegates, which are selected from the local districts.
These persons vote for anyone of their own choosing which may differ drastically from the way their own district, or state, votes. There is a lot of debate over this non-democratic method of choosing candidates, since this large number can have powerful affect on the vote. This also causes confusion when networks add the Superdelegate numbers to the various candidates in the primaries, causing all sorts of fluctuation in numbers of total delegates.
It seems that most of the superdelegates hold off endorsing until the convention, and indeed, even if they have pledged their vote to a certain candidate they can still later switch their vote. Any accounting that includes these floating pledges should be taken with a more than a few grains of salt.
At this point (list follows), only about one-third of the Superdelegates have pledged support, so cries that "they've stolen the election!" are a little premature. In addition, there are no rules governing how they vote - even if they have endorsed an candidate, they don't appear to be held by that pledge - in contrast to all other delegates.
Anecdotally, there have always been stories of delegates being sent to the convention, whereupon they switch votes. While this has happened in caucus before, one thing that lessens the effect of these "rogue delegates" is opening the process to more people, and not limiting the awarding of delegates to "big swinging dicks" and the party insiders.
Historically, there has been a concerted effort by both parties to limit voter turnout, giving the insiders and leadership greater influence on the election process. The 50-State strategy by the current DNC Chairman, Governor Howard Dean, has begun to reverse this trend, the numbers seen in voter turnouts for the primary have been astounding, with an average turnout double than in 2004.
Is it right for the Democratic Leadership to maintain the Super Delegate hold on the election? --the entire reason for the superdelegate process is to prevent the party from being affected by activists at the local level, determining the candidates, rather than the Party Leadership. So with the adoption of the successful 50 State Strategy isn't it time to revise or eliminate entirely the clearly undemocratic Superdelegate system?
Here is a pdf with a list of all of the Superdelegates