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)
"Nerds are better than other people"
"Be nice to nerds - chances are you'll end up working for one"
Often mistaken for Geeks, who aspire to become nerds, yet lack the intelligence, and end up giving nerds a bad name due to their poor social skills.
Non-nerds are often scared of nerds, due to their detailed knowledge, and therefore seemingly high levels of intelligence - and subsequently denegrate them as much as possible as often as possible.
Nerds exist covertly within the fabric of society, often choosing to 'nerd it up' in private or in the company of fellow nerds. It is for this reason they are feared the most - unlike geeks, who are easily identified, nerds can only be found out when casual conversation reaches a subject that they like nerding.
Nerd: "Let's bash this nerd"
Nerd: "That'll teach you to go playing with my tesla coil"