An arrogant person who goes into an outburst every time something he likes is questioned. Fanboys usually acuse others of being fanboys. If you insult something that a fanboy likes, he will spam your computer up and try to insult something that you like. Most words a fanboy uses are in 1337 or end with 'X0RZ' and they spell the work 'the' as 'teh' because they think that it is 'teh ro><0|2Z!!!'
e.g. Counterstrike is teh rockxorz!
Fanboys usually use the word 'own3d' if they beat someone.
Another annoying feature of a fanboy is that if he is loyal to a console, he usually goes to rival console forums and spams the place up with inults about how his console is 'better'.
They take the 'console war' very seriously, as if it's a real war.