See it frequently: What exactly is a "troll"?

I've seen forum posters here and in other audio forums accused of being "trolls" by other members. In fact, I was once accused of being a "troll". I know that it has something to do with being disingenuous and having a hidden agenda. What exactly is it?
v. trolled, trollĀ·ing, trolls
a. To fish for by trailing a baited line from behind a slowly moving boat.
b. To fish in by trailing a baited line: troll the lake for bass.
c. To trail (a baited line) in fishing.
2. Slang To patrol (an area) in search for someone or something: "[Criminals] troll bus stations for young runaways" Pete Axthelm.
3. Music
a. To sing in succession the parts of (a round, for example).
b. To sing heartily: troll a carol.
4. To roll or revolve.
1. To fish by trailing a line, as from a moving boat.
a. To wander about; ramble.
b. Slang To patrol an area in search for someone or something.
3. Music To sing heartily or gaily.
4. To roll or spin around.
a. The act of trolling for fish.
b. A lure, such as a spoon or spinner, that is used for trolling.
2. Music A vocal composition in successive parts; a round.

in chat rooms such as this, "trolling" is generally a term for starting a thread in order to promote (surreptiously, if sucessful) one's products/preferences/self, i.e. attempting to lure by stealth little fishies to your bait.
One goal of a troll is to incite a 'riot', to stir up a hornets' next. Apparently they aren't much interested in learning anything or helping anyone--they just want to make trouble.
"In the context of the Internet, a troll refers to a person who makes inflammatory or hostile comments, which by effect or design cause disruptions in discourse... The contemporary use of the term first appeared on Usenet groups in the early 1990s. It is widely thought to be a diminutive of the phrase "trolling for suckers," itself derived from the sports-fishing technique of trolling."