There are various definitions and stereotypes. A nerd can be someone who is overly intellectual pursuing subjects like mathematics, science, engineering or programming.
Nerds are often considered socially impaired, described as being shy, quirky and un-attentive to their looks.
Some are considered obsessive about non-mainstream subjects such as comic books, trading cards, science fiction and role playing.
Nerds are also the reason why you're able to surf the Internet and check status updates from your Facebook friends. They invented it, so be greatful god damnit! And if you are ever with one, get ready to have your brains fried by orders of magnitude of mental stimulation.