The meaning of backbiting and slander

We were told by Abdullah bin Ja’far al-Himyari, from Ahmad bin Muhammed bin Isa, from Hasan bin Mahboub, from Abdul-Rahman bin Sayaba, from al-Sadiq, Ja’far bin Muhammed, peace be upon them both, that he said: Indeed, it is from backbiting that you say about your brother that which Allah has concealed upon him, and indeed, it is from slander that you say about your brother that which is not in him.