Originally Posted by www.Dictionary.com

NERD –noun Slang.
1. a stupid, irritating, ineffectual, or unattractive person.
2. an intelligent but single-minded person obsessed with a nonsocial hobby or pursuit: a computer nerd.

GEEK -noun Slang.
1. a peculiar or otherwise dislikable person, esp. one who is perceived to be overly intellectual.
2. a computer expert or enthusiast (a term of pride as self-reference, but often considered offensive when used by outsiders.)
3. a carnival performer who performs sensationally morbid or disgusting acts, as biting off the head of a live chicken.

DORK –noun Slang.
1. a stupid or ridiculous person; jerk; nerd.
2. Vulgar, "Penis."