A person who lacks social skills. Drawn to anti-social hobbies (video games, reading, role-playing) as a result. Generally believe their unpopularity stems from them being smarter than the rest of us, that they'll grow up to be Bill Gates, etc. In fact, nerds are usually the least well-equipped to deal with life outside of school, and even when they do possess intelligence, still aren't smart enough to learn to express their intelligence in more acceptable ways, i.e. like all the smart people out there who aren't nerds do.
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"