Allah commands Muslims to refer to people of knowledge to acquaint
themselves with the teachings of Islam as well as all aspects of life. 

Islam assigns great importance to preserving people's honor and guarding
them against any malicious defamation and slander. It is in this light that
Islam prohibit backbiting and gives serious warning against it. Allah says:
( And spy not, neither backbite one another. Would one of you love to eat
the flesh of his dead brother? Ye abhor that (so abhor the other) )
(Al-Hujurat 49: 12) 

However, there are some exceptions for this ruling. Sometimes, it becomes
necessary to show the faults of certain people to ward off a serious evil
that may occur if they are covered. Elaborating on these exceptions Sheikh
Ahmad, a senior lecturer and Islamic scholar at the Islamic Institute of
Toronto, Ontario, Canada, states:

Generally speaking, backbiting is considered an abomination and is
therefore, haram (forbidden) for us to indulge in. Someone asked the Prophet
(peace and blessings be upon him), "What is backbiting?" He said, "It is to
speak about a person (behind his back) that which he hates (to be told about
him)" 

Although, indulging in backbiting is generally considered a sin, in light of
their detailed study of the sources, scholars have considered it permissible
to state the negative aspects or traits of a person or persons in their
absence in certain exceptional cases in order to protect the interests of
the people. Here are the conditions where it has been considered
permissible: 

1. It is not considered forbidden to mention the negative aspects of a
marriage candidate in order to help the person who is planning to get
married to formulate a sound opinion. In such a case, it becomes the
religious duty of everyone who is asked about an individual to divulge any
relevant information about him/her. 

2. A public figure who may be running as a candidate for a position is just
like a marriage candidate. Since it is vital to protect the interests of the
public, it is considered permissible to disclose such information about him
which may reflect negatively on his performance or eligibility to serve.
Public office is a position of trust and, as such, only people of integrity
should be appointed to such positions. 

3. It is allowed to mention the tyrants, despots and rebellious persons who
are role models of impiety, immorality and rebellion against Allah in order
to warn people against such behavior. It is in this spirit that we find the
Qur'an narrates to us the stories of Pharoah, Nimrod, Haamaan and Qarun, and
so on. Therefore, it is not considered sinful to speak negatively about
personalities like Abu Lahab and Abu Jahl. 

4. It is also not considered sinful to mention a person who is openly
violating the laws of Allah. It is said, "An immoral person has no honor to
protect" since he has already tarnished his honor by openly flaunting the
laws of Allah. 

5. If a person has been harmed or has been treated unjustly and unfairly by
someone, he can speak about it to the proper authorities in order to seek
justice or restore his legitimate rights. Included in this category are
those who complain to the authorities about an abusive husband, or guardian,
or leader, and so on. 

6. If a so-called leader, preacher, or scholar is known to be spreading
corrupt ideas or heretical ideas it is considered permissible to forewarn
people about him. However, in order for this to be permissible, one must
have ascertained a fact that the ideas or opinions he is advocating are
indeed heretical beyond a shadow of doubt. Otherwise, it would be a terrible
sin, for it has been rightly stated by scholars, "The flesh of scholars is
poisoned meat! 

May Allah keep us all safe from the terrible sin of backbiting, and may He
also empower us to be steadfast in performing the duty of counseling what is
good and forbidding what is wrong - Ameen.