Author: Imaam Ibnul Qayyim al-Jawziyyah (:rahim:)
Source: Ar-Ruh, Page 357
And the difference between backbiting and advice is that the intent behind advice is to warn the Muslim against an innovator or someone who can be a cause of fitnah (trial and tribulation) for him or from one who will deceive him or corrupt him.
So you will mention about him that which is necessary (from those things which were mentioned previously and or other than them) if there is between him and the other some sort of affiliation or business transaction or any type of involvement with him. As the Messenger (:pbuh:) said to Fatimah Bint Qays (:raha:) as she went to him seeking counsel concerning marriage to Mu’awiyyah (Ibn Abi Sufyaan) or Abu Jahm (:raa:), so he (:pbuh:) said to her:
“As for Mu’awiyyah then he is a poor man and as for Abu Jahm then he beats his women…” (Sahih Muslim)
And some of his (Abu Jahm’s) companions used to advise those who traveled with him that, “when you leave from his land (i.e. where he is known) then be cautious of him” (i.e. cautious of getting him married because of what he was known for of beating his women).
So, if backbiting occurs in a manner in which it is advice for the pleasure of Allaah and loving the Sunnah of) His Messenger (صلى الله عليه وسلم) and (loving) His slaves from amongst the Muslims, then it is a means of closeness to Allaah and from the hasanaat (the acts of good). And if it takes place in a manner where it is to humiliate your brother and to tear up his honor and to take delight in his flesh and to bite at it and to lower his place from the hearts of the people, then this is a disease that is incurable and it will burn up his good deeds just as fire burns up wood