Libel is a published false statement that is damaging to a person's reputation; a written defamation.
Slander is the action or crime of making a false spoken statement damaging to a person's reputation.
In this day and age of how quickly chats / tweets / statuses are shared on social media, it is pretty stupid to slam someone on social media. My advice - if you don't have something nice to say about someone, don't say anything at all. Ever. Remain silent. That way, you have nothing to worry about.