In current use, the word typically connotes an expert or enthusiast or a person obsessed with a hobby or intellectual pursuit, with a general pejorative meaning of a "peculiar or otherwise dislikable person, ecially one who is perceived to be overly intellectual".
Although often considered as a pejorative, the term is also used self-referentially without malice or as a source of pride. Its meaning has evolved to connote "someone who is interested in a subject (usually intellectual or complex) for its own sake".
Historically, a geek was a circus person in the sideshow who performed some bizarre feat. Over time, the meaning of geek has changed to include anyone with an obsession that places him outside mainstream society.