If you published something about someone, and they reckon it's defamation then you could find yourself being sued for libel.
What if the comment was true?
If you were taken to court you'd have to be able to prove that what you'd said was true. The lawyers would ask really tough questions, and expect to see serious evidence. If you couldn't prove it, and you lost the case then that could cost you a lot of money. Some people have lost everything after losing libel cases.
What if I am repeating someone else's comments?
If you read something slagging off a person on one website, and you repeated it on another site that wouldn't stop you getting done for libel. The lawyers could say that by repeating a libel you are just as guilty as the person who said it first.
But I just said it on a message board
The law says that counts as publishing, so you could be sued for libel. It's as simple as that.