The Messenger of Allah (PBUH) said: “do you know what backbiting is?” The Companions said: “Allah and his messenger know best.”
“Backbiting is talking about your Muslim brother in a manner, which he dislikes.” Someone asked “what if my Muslim brother is as I say?” Prophet Muhammad (PBUH) said: “If he is actually as you say then that is backbiting but if that is not him that is slandering.”
Both backbiting and slandering is a sin in Islam. Slandering is much worse because you are saying something bad about someone that is not true. We should not backbite or slander each other. If we talk about another one, it should always be something good.