There isn't really much differences in what backbiting is, telling things about someone that he or she would not like others to know, and if what is told is not true than that is slandering.
Abu Hurairah (May Allah be pleased with him) said: The Messenger of Allah (sallallahu 'alaihi wa sallam) said, "Do you know what is backbiting?" The Companions said: "Allah and His Messenger know better." Thereupon he said, "Backbiting is talking about your (Muslim) brother in a manner which he dislikes." It was said to him: "What if my (Muslim) brother is as I say." He said, "If he is actually as you say, then that is backbiting; but if that is not in him, that is slandering."
[Muslim].
وعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: “أتدرون ما الغيبة؟” قالوا: الله ورسوله أعلم قال: "ذكرك أخاك بما يكره" قيل: أفرأيت إن كان في أخي ما أقول؟ قال: "إن كان فيه ما تقول، فقد اغتبته، وإن لم يكن فيه ما تقول فقد بهته”. ((رواه مسلم)).
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There are also some occasions in which you are allowed to backbite, you can read here.
