Of course loshon horah is forbidden. We know it was the prime cause of the destruction of the Bais HaMikdash, and of course we know that we arent supposed to speak it--yet we do. A big part of the reason is that we dont really see it as wrong, we dont see it as damaging, and we certainly dont see the act of speaking as something that will cost us in the future.

This Shmuz is a powerful introduction to the effects that our words have on other peoples lives in this world, and on ours in the World to Come.