Below is a site link to some info on the Smith Squat..

http://www.exrx.net/kinesiology/SmithSquat2.html

And this one is bio-mechanical issues on the regular Squat..

http://www.oaktrees.org/fitness/squats.html

And this one shows photos of how to do a Smith Squat..

http://www.muscleandstrength.com/exercises/smith-machine-squat.html