Nerds are defined by what they know. We tend to stick to societally acceptable topics, but dive in much deeper or cover a wider variety of subjects than most. We are the grad students of the world, the academics, researchers and general know-it-alls.
Dorks are defined by what they like. Similar to the nerd, we dive in much deeper than the average person, but the topics we pursue tend to be much more nontraditional. We learn to speak Klingon or Elvish or know the plot lines, writers, and artists of all the major comic books and most of the minor ones.
Geeks are defined by what they can do. We may not know as much as the nerd on any given topic, but we can do more with what we know. We can hook up a home theater, fix a computer, or super-charge a lawnmower. We are the tinkerers, programmers, and garage inventors.