Nerd:
A person who gains pleasure from amassing large quantities of knowledge about subjects often too detailed or complicated for most other people to be bothered with.
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: "That'll teach you to go playing with my tesla coil"
Still angry about that but I'm getting over it.