Defamation can take the form of libel or slander, and there are clear differences between the two. Libel is written, while slander is spoken. So, if you object to something that someone writes about you on the internet, or in a newspaper, that’s potentially libel; whereas if you object to something that is said about you, that’s potentially slander.
People can say or write negative things about each other with impunity if what they say or write is true, and causes no damage to anyone’s reputation. If it’s true, and no reputation is hurt by it, then it doesn’t matter how much anyone objects to what has been written or said, because they’d have no basis for a legal case.
For example, if you were to write “Jane Smith is a big fat lump” I couldn’t do anything legally without making myself look both fat and foolish because it’s true: I am a big fat lump, no matter how much I object to anyone pointing that out. However, if you were to write “Jane Smith is a criminal” then you might be in trouble: I’ve never been convicted of any offence, and would therefore have grounds to sue you—but only if I were to experience a loss as a result of your comments.
It’s not just people who can sue on grounds of libel or slander: any entity or organisation with a reputation to protect can be defamed. So corporations can sue people who say nasty things about their businesses, for example, if those nasty things are untrue and cause the corporation a legitimate loss.
And remember, I’m not a lawyer, just a writer who has made it a priority to find out about these things. If you’re considering your own defamation case then get yourself proper legal advice, and don’t rely on anything you hear from a big fat lump like me.