Pistols are 1 leg squats.
http://www.powerathletesmag.com/pages/pistols.htm

I've started them by doing them down onto a low chair - like box squats. I can't go further at a decent (slow) speed and maintain control and balance.
The first time I tried them onto the chair, I only did a few with each leg, as a test run... and for the next few days my glutes (maximus) were very sore, like the kind of sore that is DEEP and lasts.