working within a tech field we have a nice assortment of nerds and geeks, so one day, being the geeks that we were, we developed a strict level of classification for each, and they are as follows:

nerd: someone who is obsessively knowledgable of a practical/useful field, such as computers.

geek: someone who is obsessively knowledgable of a completely useless field, such as role playing games.