Geeks are focus. They master everything about their particular madness. Nerds use their knowledge widely. Yet the terms can be use for the same person. A geeky nerd can use his skill @ D&D anywhere in real life AND MAKE IT WORK!
A nerdish(?) geek uses skills from everywhere to make his madness even more epic. Once you know a geek you can usually guess where it is coming from, nerds will always shock you as to where there got their powers (not necessarily what they con do).