I've always been, well, a geek. Some would say I've always been a nerd. It's semantics.
Nerd is defined by the dictionary as "an intelligent but single -minded person obsessed with a nonsocial hobby or pursuit." Geek can be defined similarly or as Wikipedia says "it's changed over time and there is no a definitive definition."
I tend to think of a geek as someone quirky, maybe focused on one subject to the exclusion of all others. I was definitely a book geek growing up. I read all the time.
When I think of nerds, and I know it's wrong, I think of the Revenge of the Nerds movies and the stereotypical character featured in those movies. A lot of people don't see that stereotype and refer to themselves as nerds without distress but that is probably why I prefer the term geek.
Oddly enough, my children refer to themselves as geeks and are proud of that fact. So do several of oldest's friends. A recent Phineas and Ferb episode had them referring to themselves as nerds and proud of that. The titles now seem to be worn as badges of honor.
I love it!
When I was growing up, while I was a nerd or a geek, it wasn't something accepted. Hence the Revenge of the Nerds movies among others. Today's society seems to be better accepting of the geek/nerd phenomena or at least in the circles I'm moving in right now.
I married a computer/science fiction geek who has led me even further down the path of geekdom. Geek Love and Code Monkey sprung from that those two stories are near and dear to my heart.
It's a brave new world. Geeks rule!