Slander and gossip, including their laws
Lashon Hara (slander, literally "the evil tongue/language") is forbidden by the Torah. It refers to any sort of derogatory communication, true or not, about a third party. The many intricacies of its laws are most authoritatively laid down in the book Chafetz Chaim by Rabbi Yisrael Meir Kagan of Radin.
Also forbidden is Rechilut (gossip, literally "tale-bearing") which is not derogatory but which evokes ill-feeling about the subject in the listener. This tag includes both, and Motzi Shem Ra (roughly, false Lashon Hara).