The difference between advice and backbiting is that the intent behind advice is to warn the Muslim against an innovator, deceiver or an evil doer.
So you will mention about him that which is necessary if one was to seek counsel from you about his companionship, having business transactions or any type of attachment with him as the Messenger صلى الله عليه و سلم said to Faatimah Bint Qays when she went to him seeking counsel concerning marriage to Mu’aawiyah (Ibn Abi Sufyaan) or Abu Jahm, so he صلى الله عليه و سلم said to her:
“As for Mu’awiyyah then he is a poor man and as for Abu Jahm then he used to beat his women.“
…(as explained in a narration in Saheeh Muslim: Hadeeth: 1480-47. The chapter of divorce), and some of his (Abu Jahm’s) companions used to advise those who traveled with him that when you leave his land (where he is known) then be cautious of him (i.e. be cautious of marring him because he is known for 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 و سلم and (loving) His slaves from amongst the Muslims, then it is a means of closeness to Allaah and from 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 a fire of hassanaat (good deeds) that burns them just as fire burns up wood.
[Taken from: Ar-Ruh (The Soul) by Ibnul Qayyim: P 357]