The art of deliberately, cleverly, and secretly pissing people off, usually via the internet, using dialogue. Trolling does not mean just making rude remarks: Shouting swear words at someone doesn't count as trolling; it's just flaming, and isn't funny. Spam isn't trolling either; it pisses people off, but it's lame.

The most essential part of trolling is convincing your victim that either a) truly believe in what you are saying, no matter how outrageous, or b) give your victim malicious instructions, under the guise of help.
Trolling requires decieving; any trolling that doesn't involve decieving someone isn't trolling at all; it's just stupid. As such, your victim must not know that you are trolling; if he does, you are an unsuccesful troll.

Signs that your trolling is succesful:
*Your victim screaming in all-caps at you.
*Personal attacks (Calling you a retard, idiot, etc).
*Being an Internet Tough Guy.
*Making a crude remark, before quickly logging off before you can retort.

Signs that your trolling is unsuccesful:
*Your victim identifying you as a troll.
*Identifying yourself as a troll.
*Your efforts being ignored.
*Being counter-trolled (See below)

http://www.urbandictionary.com/define.php?term=trolling