A generally intelligent individual who has an in-depth interest (or interests) in a particular area, far more than the average joe. They are typically cast as having a lack of social skills and "no life", whatever that may mean. All of these statements may or may not be true, depending on the person.
Bill Gates once observed that you should "be nice to nerds. You'll probably end up working for one one day", or a similar phrase.
Nerds either do or do not share one or more characteristics with geeks, depending on who you ask.
They also seem to be somewhat bitter at the possibly unjust typecasting they receive on behalf of society at large. Nerds generally get victimised by society for their difference, and so seem to feel the need to fight back by "reclaiming the term" to mean something good. Their general (but by no means pervasive) lack of social skills means that much of this "nerd and proud" vitriol is vented on the Internet and nowhere else.
(see other definitions for proof of this)
"That guy spends all day on the computer. He's such a nerd"
"Nerds are better than other people"
"Be nice to nerds - chances are you'll end up working for one"