Backbiting is defined by a Hadeeth narrated from the Prophet Muhammad (SAWA) as : Mentioning your brother by a matter which he dislikes to be mentioned. A person asked the Prophet: Even if I mention a real matter in him? The Prophet replied: If you mention a real matter which does not like to be mentioned, yoy have committed backbiting him, and if you mention a matter which does not exist in him, you have put allegation on him.
Allegation is obviously more dangerous major sin.
Backbiting is a bad habit in any way. Backbiting a believer is a major sin. Backbiting any one is a bad act unless you want to warn people from a possible harm from a person.