Geek - socially functional person. A fan of popular sci-fi, computers/gadgets. Not limited to popular or 'cult' movies/music and/or consumer products, also may include sports and fantasy football leagues and cars. Leonard Hofstadter from Big Bang Theory.
Nerd - barely functional in the herd society. May focus on one hobby or cultural phenomenon and discard all others as inferior - and is quick to point out that his/her opinion is the only one worth discussing. Sheldon Cooper.
Dork - social misfit, unable to clue in or conform to many of the herd's cues. May also have a mild deformity and/or hygiene issues. Napoleon Dynamite.