A troll, is someone who tries to harass by posting messages on the internet. The purpose is to excite strong emotion in an online community. This is to get a reaction from other online users, or to cause problems.
Sometimes trolling can get a user banned as it can be disruptive. Some users do it to be humorous when they know others will be angry. The best solution is to ignore trolls and to not give them the attention they want. This has become known as "don't feed the trolls".
Troll (internet) Facts for Kids. Kiddle Encyclopedia.