What is the difference between backbiting (نميمة) and slander (غيبة)? And is describing people's appearances in their absence considered backbiting, even if the intention is for differentiation or identification and not to cause harm?

Backbiting is mentioning a Muslim about something he dislikes in his absence, and tale-bearing is conveying speech to cause mischief among people. Both are forbidden (haram). Mentioning a person by their description or appearance is considered backbiting if they dislike it, even if the intention was not to offend, based on the Prophet’s (peace be upon him) saying: "Your mentioning your brother with something he dislikes." However, if he does not dislike it, or if it is for the purpose of identifying and distinguishing him out of necessity, then it is not considered backbiting. Backbiting may become obligatory or permissible for a legitimate Sharia purpose that cannot be achieved except through it. Six cases are exempted from backbiting: the oppressed, the introducer, the warning person, the one who openly commits sin, the one seeking a fatwa, and the one seeking help to remove an evil.