Traditional meanings of this word, used either as a noun or verb, are that of a Scandinavian cave dweller (in folklore), the act of singing out in a jovial manner, fishing using a baited line drawn along the water or simply searching for something.
However, new uses of the word have developed via the Internet, and also in the popular press which have added to the above meanings.
A troll is now also used to mean someone who deliberately posts either anti-social or defamatory comments on-line with the intent of provoking reaction and causing shock or controversy.
Trolling is the act of committing such violations, purely in order to gain an emotional response and to cheese other people off.
Some suggested examples of use can be found at the Urban Dictionary.
There is also some scientific evidence that trolling acts on groups emotionally and serves to polarise opinion, as seen in this article in The Independent.