There are quite a few mitzvos that involve gossip and slander. Collectively, such evil speech is known as lashon hara (which literally translates as “evil speech”). This particular form of smack talk, however, is called rechilus – “talebearing” or gossip.
The nature of rechilus is as follows: Jane hears Mary say that Sally is nasty. Jane runs to Sally and says, “You should hear what Mary is saying about you!” Now, not only is Mary badmouthing Sally, Sally is mad at Mary. Jane, the gossip, has increased animosity among members of the community rather than defusing the situation.
The word rechilus comes from rachil, a peddler. The gossip is like a salesman, going door to door hawking their merchandise. Unfortunately, in this case, the merchandise isn’t brushes or encyclopedias, it’s slander, anger and hatred.
Rechilus is a particularly dangerous thing. The Talmud in Arachin (15b) says that it metaphorically “kills” three people – the speaker, the recipient and the subject. Therefore, it behooves us to stay far away from this matter, even as the seemingly-innocent listener!
The reason for this mitzvah is that God wants there to be peace among His children, not animosity. Plain old slander is bad enough, but spreading it around only makes things worse!
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in the tractates of Arachin (15b), Kesubos (46a) and elsewhere. It is codified in the Mishneh Torah in the seventh chapter of Hilchos Deos. This prohibition is #301 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #77 of the 194 negative mitzvos that can be fulfilled today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.
As an aside, the Chofetz Chaim’s main “stock in trade” was educating people about gossip and slander. Aside from the Sefer HaMitzvos HaKatzar and the Mishnah Brurah, the Chofetz Chaim wrote many other works. At least three of these are dedicated to various forms of lashon hara: Shmiras HaLashon, Chovas Hashmirah, and Chofetz Chaim, the book from which his well-known nickname is derived.