The Prophet (peace and blessings of Allah be upon him) explained the meaning of an aayah.

He said: “Do you know what gheebah (gossip or backbiting) is?
They said, “Allah and His Messenger know best.”
He said: “To say something about your brother that he does not want to be said.
He was asked, “What do you think if what is said about him is true?”
He said, “If what you say about him is true, this is backbiting, and if what you say about him is not true, this is a lie.
(Reported by Muslim, 4/2001)..

Gossip or backbiting means saying something about a Muslim which may be true but which he does not like to hear spoken, whether it be about his physical appearance, his adherence to religion, his worldly affairs, his self, his behavior or his character.

There are many forms of gossip, including talking about a persons faults and imitating him to make fun of him.

People take the matter of gheebah very lightly, although it is very serious in the sight of Allah, as is indicated by the words of the Prophet (peace and blessings of Allah be upon him):

There are seventy-two forms of riba, the least of which is as bad as a man having intercourse with his own mother, and the worst of which is when a man slanders the honor of his brother.”
(Silsilah al-Saheeh, 1871).

(peace and blessings of Allah be upon him) encouraged us to do this when he said:
Whoever defends the honor of his Muslim brother, Allah will protect his face from the Fire on the Day of Resurrection.”
(Reported by Ahmad, 6/450; see also Saheeh al-Jaami’, 6238).