It is a fairly new device that you plug into your wall socket at home, and it releases continuously a flow of pheremones into the room, calming down excitable cats, stopping confrontations and preventing problem spraying.
You can find it at any good pet product store, or online you can order it from Farnum Pet
I use the feliway plug in and it helps a lot. Want to get another one for the attic now, since Tilly insists it is HER attic, and Pan says that it's HERS. They have had *words* over the past two days about this.