Often used interchangeably with nerd or geek.
Dorks are typically more noted for their quirky personality and behavior rather than their interests or IQ which may or may not be on level with traditional geeks or nerds. They tend to be more humorous and extroverted and don't mind laughing at themselves or with others at themselves, as the case may be.
After the 1990s, the term dork tended to specifically refer to a person who often shared the characteristics of geeks or nerds but were not ostracized as a result. Also, while old school geeks and nerds tend to continue to accept an "outsider" status and maintain an elite club mentality amongst themselves, dorks generally tend to do the opposite, hence a current preference with the mainstream for dorks over geeks or nerds.
He may a dork but he is fun to be around.