Backbiting is a major sit which mentioned in Quran as eating the flesh of your dead brother. (And do not backbite each other, does any one from you like eating the flesh of his dead brother, then you will hate him).
Backbiting is (Mentioning any Momin with anything which he / she does not like to be mentioned). If it was a true, then it s backbiting, but it was untrue then iy will be allegation which more dangerous.
Repenting from backbiting is Must otherwise it will lead to hellfire.
As it is a sin concerning others, we must seek pardon from them and request them to sincerely forgive us. If this is not possible either if the person is no more alive or he is not contactable a all or it might be more harmful to inform him, then we need to pray for him and gift him good deeds which will make him in the Day of Judgment forgive us.