Backbiting is speaking slanderously about an absent person. The Prophet sallallahu alaihi wa sallam said:
"Do you know what is Gheebah?" They said: "Allah and His Messenger know best." He said: "It is to attribute to your brother what he dislikes." He was asked: "What do you think if what I say about my brother is true?" He said: "If what you attribute to him is true, then you have backbitten him, and if it is not true, then you have lied about him."
What is Nameemah?
Nameemah is to circulate slanderous rumours between two persons to damage or sever the ties between them. The Prophet sallallahu alaihi wa sallam said:
"No Nammam will be admitted to Jannah".
Source: Pg: 22 Guidelines for Raising Children, Darussalam