Lashon ha'ra is saying (or communicating in any way--even by rolling your eyes, winking, etc.) anything derogatory or negative about someone that can hurt him or her in any way--by causing embarrassment, loss of money, lowered esteem, bad reputation, etc. 
 
Lashon ha'ra in which you attribute the negative statement about the person to whom you are speaking to someone else is called rechilut (gossip). 
 
Lashon ha'ra that is false is called motzi shem ra. Saying something that is true is not lashon ha'ra if it is being said for a positive purpose (even though the same words would be lashon ha'ra if they were only intended to hurt another person and had no positive purpose).
 
Lashon ha'ra, including rechilut, is forbidden to be said about shomer Shabbat Jews except for a positive purpose; motzi shem ra is forbidden to say about anyone--Jewish or not, shomer Shabbat or not, whether for a positive purpose or not.
 
Speaking Lashon HaRa
 
When You May Say Lashon HaRa
Lashon ha'ra, including rechilut, even if truthful, is not permitted except for a constructive or positive purpose. (Motzi shem ra is never permitted.) Its purpose cannot be to hurt a person.  It is permitted and recommended to tell the facts about someone:

1. To protect others from being hurt (being cheated, molested, etc.), or
2.  For a positive purpose that cannot be achieved through any other means.

 
Lashon HaRa To Protect Others from Being Hurt
You may say truthful lashon ha'ra (including rechilut, if it is necessary to divulge the name of the person who told you):

• To prevent a bad person from telling children bad things or to keep children from learning bad behavior from the bad person;
• To avoid being implicated in what the bad person is doing;

• To prevent other Jews' suffering a loss by using a worker who did bad work for you.

• If a businessman cheats you or lies to you, you should warn other Jews about him (but only if you suspect they want to do business with him).
• If you know something bad about someone who a third person wants to date or marry, in many cases you are required to tell what you know (but this can be very complicated and dangerous and a rabbi should be consulted about what to do in many cases)!

 
When You May Add Facts to Correct Possible Lashon HaRa
If the reasons behind an action are not clear and someone may get the wrong/negative impression of someone in question, then you should tell the facts and tell the entire story. 
Example
Someone (A) insults another person (B) in public. Entire story is that B beat up A previously. Knowing the full story changes how people might view A.
Note If it is only your opinion, state that instead of declaring it as fact.
 
Lashon HaRa To Allow Gain
Saying lashon ha'ra for a positive purpose includes gain for yourself or for someone else; you may talk about others in cases such as these:

• Psychotherapy  You may say truthful lashon ha'ra or rechilut to a psychotherapist since you don't know what is important. Outside of therapy and with non-therapists, you may say anything that will bring about a positive result (and only if there is no other way to achieve that result).

   

• Upset  If you are upset by what a person did to you and it will help you to calm down by telling what was done to you (that is, you will gain by feeling better).

 
Note If you want to ask someone for information that could be lashon ha'ra, you should say why you are asking so the other person will understand that it is for a positive purpose and is therefore not lashon ha'ra.
 
 
Lashon HaRa and Specific Cases
 
Children, Shomer Shabbat Person, Groups
Even truthful lashon ha'ra, including rechilut, may not be said about a Jewish child or a shomer Shabbat person or group of people unless for a positive purpose that cannot be achieved any other way.

• A parent should not rebuke or criticize a child if it will embarrass the child in front of others.
• A child (whether young or adult) may not correct a parent who is saying lashon ha'ra, unless the parent would want to be reminded that the parent is saying lashon ha'ra. Even then, it must be done respectfully. 
• You may make statements about groups of people in general, even if negative, as long as the purpose is to protect other people from them. But what you say must be true.   

 
Lashon HaRa: Public Knowledge
You may mention information that is public knowledge. But your intent should not be to spread the word, but rather just to pass along interesting information.
Example
Saying, “Did you hear that the president of the synagogue just got convicted of...”
 
Lashon HaRa: Mass Media
You may read in the paper, see on TV, or hear on the radio an account of someone's bad behavior (since you cannot know whether it will affect you or be important for you to know until you read or hear the information, it may be OK; ask a rabbi). You may not accept it as being the complete truth, but you should be wary.
 
Lashon HaRa: Shomer Shabbat Public Figure
You may not tell or listen to lashon ha'ra about a public figure who is a shomer Shabbat Jew unless there is a purpose.
You may give your opinion about a shomer Shabbat politician as long as you state it as your opinion and as long as giving your opinion may help other people.
 
Lashon HaRa: Shomer Shabbat Organization
You may not say any of the three types of speaking ill about an organization, school, synagogue, etc., whose members or employees are shomer Shabbat--except for a positive purpose.
Examples
You may not say any of the three types of lashon ha'ra about a Jewish school's cost, bad teachers, etc., unless it may be relevant to future students (and even if it is relevant, you may still not say motzi shem ra).
You may not say, “I don't like that shul because there is lots of talking,” unless you think the person will appreciate knowing since he or she will not want to go to a shul with lots of talking.
 
Lashon HaRa: Asked for Opinion
If you are asked for your opinion, you may give it if it is relevant (has a useful purpose) to the person asking. Otherwise, no comment.
 
If you are asked your opinion about a Torah lecture or lecturer, you may give your opinion only AS your opinion, not as criticism.
 
Lashon HaRa: Told in Confidence
If you have been told something in confidence, even if you think it is best for the person who told you if you pass it along, you may still not re-tell anyone else. You may say, “I cannot speak about that” if you are asked. Consult a rabbi.
 
Lashon HaRa: Getting Back at Someone
If someone hurts you, you may get back at the person at the time of the action against you.  But afterward, it would be revenge and is forbidden.
Example
Someone tells you, “You are good for nothing.” You may say, “You are worse than I am,” if it makes you feel better.
 
Lashon HaRa: Bet Din Summons
If someone with whom you have a problem refuses a hazmana (summons) from a bet din, you may publicize a letter from the bet din saying the person refused the hazmana so that the recipient might agree to go to the bet din due to public embarrassment.
 
Lashon HaRa: Bet Din for Abuse
All matters between Jews should, ideally, initially go to bet din but only if the bet din is capable of resolving the problem. In cases of suspected child or spousal (or other) abuse, you may report it to the police if there is no bet din that is capable of dealing with the problem immediately: You are not required to wait while a bet din gets around to your case. The key is to expedite the case.
Note Some cities have special batei din for such matters.
Note Beware of governmental agencies that may take away children from their homes, even without evidence.
 
Listening to and Believing Lashon HaRa
Listening to and believing any category of lashon ha'ra is also forbidden. If you do hear something bad about someone else, do not believe that it is definitely true--but you may believe that it might be true. When you hear lashon ha'ra:

• You can try to change the subject, since pointing out that lashon ha'ra is being said may not stop it from being said.
• If you see a shomer Shabbat Jew doing something that seems to be forbidden, you should judge him/her favorably and assume that there is a good interpretation to what is being done.

Example
Situation
You see someone who is not shomer mitzvot get in a car on Shabbat.
What To Do
Make the logical assumption that he/she is going to drive (and not for a halachically permissible purpose).
Situation
You see a shomer Shabbat person get in a car on Shabbat.
What To Do
Assume there is a good, halachic reason for it. However, you may not ignore reality or make implausible or unlikely excuses for bad behavior.

You must tell whatever you know that is relevant about a potential date that someone is considering marrying.