Assalaamu alaikum Legendary,
Slander, as has been explained by other members, is saying damaging things about another person that are not true. Backbiting is saying damaging things about another person that are true, but that you would not say in front of them, and that they would not want others to know. For instance, you went gambling, and your friend caught you at it, and then went and told everyone. This would be backbiting.
However, there are exceptions. For instance, if you stole some money, and your friend came to know about that, it would be his duty to tell the person you stole from (unless you were decent enough to repent and go and give the money back yourself; then your friend should keep quiet), because you have caused harm to another. And if you had a habit of stealing, then it would be his duty towards the larger community to let people know your character, so that they might protect themselves (and in most cases, this is most appropriately done through the legal authorities). But we must never be quick to disclose the faults of others. It should always be done reluctantly, and for a good and legitimate purpose.
(smile) And I would suggest we disclose only after sincerely asking Allah to Guide us in our decision.
However, you can talk about others when they are not there in a positive way. If you want to share something good that another person has done, I don't believe there is any problem with that. Indeed, it might cause some good, both for the person, and for the general community, if it can build trust and love.
(smile) But only Allah truly Knows what is best, and what are the inner workings of our hearts.
May Allah, the Oft-Forgiving, Guide us to choices and actions that Please Him.