Trolling is trying to get a rise out of someone. Forcing them to respond to you, either through wise-crackery, posting incorrect information, asking blatantly stupid questions, or other foolishness. However, trolling statements are never true or are ever meant to be construed as such. Nearly all trolled statements are meant to be funny to some people, so it does have some social/entertainment value.
"Trolling" isn't simply "harmful statements". Intentionally insulting/libelous statements are "flaming".
Just as bad as trolling is "Feeding the Trolls". This is when people say stuff that they know will prompt someone to respond with a trolled reply and/or replying to comments that are blatantly from a troll. This is especially true when a troll first makes his comment/reply, and (usually many) people respond, either trying to correct the troll, or express anger at the statement. At that point, the trolling was successful and has been fed. When encouraged by success and feeding, trolls often return.
In World of Warcraft, there are occasionally people that blurt out in world chat, "You get your mount at level 40 and costs 100 gold." then immediately signs off.
This information is outdated by over a year; but MANY people respond for at least 10-15 minutes afterwards with either corrections or calling that player a 'dummy', or something similar insulting their intelligence.
Trolling and subsequent feeding. Perfect example.