I know that in principle, backbiting is prohibited but I want to know of differences of opinion about what exactly is considered backbiting.
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."
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)).
There are also some occasions in which you are allowed to backbite, you can read here.
The Prophet said backbiting is Your mentioning something about your fellow brother that he would not like.
A hadith of the Prophet says:
10ـ رسولُ اللهِِ (صَلَّيَ اللهُ عَلَيهِ وَ آلِهِ) ـ لأبي ذرّ ـ: .... يا أباذرٍّ، إيّاكَ والغِيبَةَ؛ فإنَّ الغِيبَةَ أشَدُّ مِن الزِّنا ... قلتُ: يارسولَ اللّهِ، وما الغِيبَةُ؟ قالَ: ذِكرُكَ أخاكَ بما يَكرَهُ، قلتُ: يارسولَ اللّهِ، فإن كانَ فيهِ ذاكَ الذي يُذكَرُ بهِ ؟ قالَ: اِعلَمْ أ نَّكَ إذا ذَكَرتَهُ بما هو فيهِ فقدِ اغتَبتَهُ، وإذا ذَكَرتَهُ بما ليسَ فيهِ فَقَد بَهَتَّهُ.
10– The Prophet (SAWA) said to Abu Dharr, ‘O Abu Dharr, beware of backbiting, for verily backbiting is worse than adultery…’ [Abu Dharr narrates], I asked, ‘O Prophet of Allah, and what is backbiting?’ to which he replied, ‘Your mentioning something about your fellow brother that he would not like.’ I asked, ‘O Prophet of Allah, and what if the thing mentioned about him was actually true about him?’ so he replied, ‘Know that if you say something about him that is true then you have indeed backbit about him, and if you what you have said is not true, then you have indeed slandered him.’
Source of Hadith: [ Bihar al-Anwar, v. 77, p. 89, no. 3]