The following comes from the Mayo Clinic website:
Narcissistic personality disorder is a mental disorder in which people have an inflated sense of their own importance and a deep need for admiration. They believe that they're superior to others and have little regard for other people's feelings. But behind this mask of ultra-confidence lies a fragile self-esteem, vulnerable to the slightest criticism.
This definition sure seems to describe a fair number of people of my acquaintance! I read where someone described narcissism as a very labor-intensive sort of condition: in short, its exemplars find it necessary to spend vast amounts of time and energy reinforcing their aggrandized image of self at the expense of, shall we say, "healthier" avocations and relationships in general. Someone else called these people "emotional vampires" -- I'd probably use a more graphic metaphor to describe them, but it serves to illustrate why they should be avoided, even if some of them are victims of a mental disorder that possibly is beyond their control.
[Update 5/19/10: I was just thinking about how people in the "mainstream" (i.e., leftoid) press were accusing Sarah Palin of having NPD after I wrote this post. I've never seen a clearer case of projection in my life -- if anything, it's the media who think they're superior to others and whose self-esteem crumbles under the slightest criticism.]
[Update 12/1/10: I read today that NPD, along with four other personality disorders previously recognized as such, will be omitted from the new DSM-V when it's published in 2013; I guess narcissism is the new "normal." Jim Taranto of the Wall Street Journal's "Opinion Journal" website has implied that he believes the timing of this omission is not coincidental given the current occupant of the White House. All I know is that the vast majority of the people I know who display the characteristics of NPD have significantly left-of-center political viewpoints, and they certainly regard themselves as normal, however nutty and unpleasant they appear to others.]