Under Democratic Party rules, all sitting Democratic governors, members of Congress, and members of the Democratic National Committee, and all sitting or former Democratic presidents, vice presidents, congressional leaders, and DNC chairs, enjoy automatic delegate status at the national convention. The creation of these "superdelegate" positions in the 1980s was designed to give the party's leaders some potential influence in the selection of the presidential nominee and approval of the party platform, as well as a formal participatory role in the convention every four years.
In 2008, the margin between Barack Obama and Hillary Clinton in the pledged delegate count (those delegates allocated based on the results of primaries and caucuses) was sufficiently narrow that the superdelegates could have decided the nomination, mathematically speaking. In practice, however, any suggestion that the outcome among pledged delegates (which favored Obama) be reversed by the superdelegates was met with concerns—and even threats—that denying the nomination to the pledged-delegate winner would fatally split the party. Then-House speaker Nancy Pelosi went on record in March 2008 as questioning the legitimacy of such a turn of events, and enough superdelegates endorsed Obama on the last day of the primary calendar that his path to the nomination was cleared—prompting Clinton to quickly concede the race.
This time around, Clinton's massive lead in superdelegates over Bernie Sanders initially suggests a built-in advantage for her campaign, forcing Sanders to win a supermajority of pledged delegates in order to make up the gap. In the wake of the New Hampshire primary on Tuesday, some news outlets even reported that Sanders's landslide victory only netted him a tie in the state delegate count after factoring in the New Hampshire superdelegates (such as Senator Jeanne Shaheen and Governor Maggie Hassan) who have publicly endorsed Clinton.
But it's once again difficult to imagine the superdelegates actually working to deny Sanders the nomination if he places first in the pledged delegate count—even if most of them prefer Clinton or view her as a superior general election candidate. The norms of internal "democracy" within the party organization are sufficiently strong that the legitimacy of a Clinton nomination under such conditions would be widely contested. A firestorm would ensue that would extend all the way to the convention itself, if not beyond, tearing the party apart and significantly weakening the eventual nominee.
It's worth remembering that the current nomination system is itself the product of a crisis of legitimacy. After Lyndon Johnson withdrew from the 1968 presidential race, the Democratic primaries were dominated by anti-Vietnam War candidates Gene McCarthy and Robert Kennedy, but Hubert Humphrey captured the nomination on the first ballot amid protests and violence both inside the convention hall and on the streets of Chicago (Kennedy had been assassinated by the time of the convention, but still would not have been nominated had he lived). The requirement that most delegates be selected by party voters rather than organizational leaders was adopted in response to the criticism that Humphrey's nomination did not reflect the anti-war sentiment of the Democratic rank-and-file that year.
So I would argue against paying too much attention to the superdelegates, whether you're a Sanders supporter angered by their lack of support for your candidate, a Clinton backer who views them as a reassuring backstop to your candidate's currently shaky campaign, or a neutral observer just trying to make sense of the process. The winner of the pledged delegate count will almost certainly be the 2016 Democratic nominee—or else the Democratic Party will really feel the burn.