Being a rather old-school geek, I have a pretty specific definition of "nerd", and it's a very different definition than the one for "geek".
A nerd generally has to be absorbed into their science/tech/art of choice to the point that they can't easily talk or relate to any person who doesn't share their obsession. They're the type who will string the jargon of their hobby/job/obsession into daily speech, and look down upon anyone who doesn't understand said jargon. A geek may share the interest, and even be capable of communicating effectively with the nerd... but the geek will have other interests as well, and be capable of (possibly broken) communication with the outside world.