A "dork" is an uncool person. It's someone who wears clothes that aren't in style, who does things that are silly, or who has really specialized interests that other people might think are boring. You can call your close friend "a dork" if they do something that's fun but not quite mainstream. For example, if your friend goes to a comic book convention, you can tease him by saying:
You're such a dork!
The word "dork" is slightly negative, but in a fun and harmless way. If you call a stranger a "dork", they might get mad at you. But if you call a close friend a "dork", they'll probably laugh and insult you right back.
The word "dork" is similar in meaning to "nerd" and "geek". There are very small differences in the meanings of these words. English speakers like to discuss the differences between them, but there's not a clear-cut difference and they're all used in mostly the same situations.