Definitions: Gossip, Backbiting, and Slander
Talking about others includes three categories, namely gheebah (backbiting) , nameemah (gossip), and buhtan (slander).
Backbiting is to say things about people in their absence, even though these things might be true, that they would not like to hear about themselves. Gossip means passing on the news of people with the intention of causing them harm or gaining a desired reward from those to whom the news is conveyed. Slander is to spread false information or lies about someone.
About backbiting, Allah says in the Qur'an what means:
*{O you who believe, avoid much suspicion, for some suspicion is a sin. Neither spy on one another, nor backbite one another. Would one of you love to eat the flesh of his dead brother? Nay, you would detest it, [so similarly, avoid backbiting]. And fear Allah. Indeed, Allah is Most Forgiving, Most Merciful.}* (Al-Hujurat 49:12)
This verse shows us how serious backbiting is in Islam. Allah also shows us how we fall into this sin. First we become suspicious of others and begin to keep track of what they are doing and saying.
While we do this, we interpret their actions and speech in our own way, which often is inaccurate because we do not know all of their motives and intentions. Then we start to talk about them in their absence, often saying things about them that are not true, and if they knew what we were saying about them, they would be upset a great deal. Sometimes our suspicions and interpretations are correct, but this does not mean that it is a good thing to mention these things to others.
For example, if we were monitoring a brother's actions and found him to be speaking to a young lady whom we do not know, we might assume many things. She may be his sister, a cousin, or a neighbor. She may be a classmate, an employer, or a coworker. She may be someone he is considering to be a prospective wife for him, or he may have bad intentions in his interaction with her, God forbid.
For us to assume one or more of these things to be true, without ever asking him personally, and then to go around spreading our suspicions among others, we run the risk of needlessly damaging the reputations of both of them. If our worst suspicions are true, then this is also something that should not be spread to others. He should be given advice privately to be careful to not follow on something that may lead to wrong so that they do not fall into sinful behavior and their reputation are not tarnished.
The Prophet Muhammad once asked his Companions, "Do you know what is backbiting?" His Companions said, "Allah and His Messenger know best." He said, "Backbiting implies your talking about your brother in a manner that he does not like." It was said to him, "What is your opinion about this if I actually find that shortcoming in my brother?" He said, "If what you assert (of a shortcoming) is actually in him, you have backbitten him; if it is not, you have slandered him" (Muslim).
More Listening, Less Talking

We can avoid sinful use of our tongues by limiting the type of things we talk about as much as possible. We should spend more time listening to others than we do talking about others. After all, Allah gave us two ears and only one tongue!
If we find ourselves in the company of those who we know to be the gossipy type, we should try to get them to change the topic of discussion. If we are not able to do so, we should leave before we get involved in gossip or talk about useless things.
Repentance and Forgiveness
One who commits a sin, whatever it is, should have hope in Allah's mercy. They must be sure that Allah will accept their repentance. We are told in the Qur'an that:
*{Say: O My [Allah's] slaves who have been prodigal to their own hurt, despair not of the mercy of Allah, Who forgives all sins. Lo! He is the Forgiving, the Merciful.}* (Az-Zumar 39:53)
In order for repentance to be accepted, they must desist from the sin they have been doing, regret having done that sin, and make firm and sincere intention not to return to that bad deed again.
Scholars add that if the sin involves a human's right, repentance requires a fourth condition: to be absolved from such a right. If it is a property, the sinner should return it to its rightful owner. If it is slandering or backbiting, the sinner should ask pardon of the offended.
Therefore, try to seek the forgiveness of the person you have backbitten. However, some scholars say it is not necessary to go to the offended (especially if they do not know you have slandered or gossiped about them) and seek their forgiveness as this might cause harm and create hatred between them.
If you cannot reach that person, make lots of supplications to him or her whenever you remember him or her.
Controlling Words Once Spoken: Mission Impossible
Once our words come out, they get bigger and bigger. Even if we regret them and wish to take them back, we would not be able to. Once the word is out, it is out and not easily forgotten or forgiven by others. The damage is done. So it is absolutely crucial that we choose our words very carefully before we let them out into the world.
We should ask forgiveness in this life of those we hurt so that we can avoid a more difficult repayment of our sins in the hereafter. If they are not able to forgive us in this life, Allah will give them their rights on the Day of Judgment by taking from our good deeds and giving their value to those whom we wronged in this life.
We should try our best to set things right in this life so that we can face Allah in the best possible state we can. The Prophet told us that whoever guarantee their tongue, the Prophet guarantees Paradise for them (Al-Bukhari) .
I hope this has been beneficial to you. I pray to Allah to help all of us to consider our words carefully before we speak them so that we may stand before Him on the Last Day with a pure and clean heart, amen.