"Ass is king."
The squat has always been referred to as a core excercise movement to get a nice strong butt and legs, but it really is more than that. In the squat there are numerous muscles of the body working in unison to provide the stability and mobility needed for this movement. It has been suggested that there are up to (and possibly more than) 200 muscles involved in this lift.
One question that is often asked is, "how do squats promote growth throughout the body beside the big strong ass?" To answer this requires more than simply saying that it's a compound movement. It involves multiple joints and muscles which in turn increase the level at which the nervous system must coordinate movement in conjunction with the lifter's muscle-skeletal system.
Exercises like the squat can produce a dramatic effect on the body by increasing, among other things, one's hypertrophy and maximal strength. The body is complex and integrated with many of its systems interwoven with each other and working synergistically to produce an effect.
The nervous system, muscle-skeletal, and hormonal systems are responsible for the effects promoted by exercises like the squat. There are various hormones which produce this effect, and the one that most people are familiar with is testosterone.
Squats can increase growth throughout the body because they use numerous muscles and this means they stimulate more muscle fibers than, say, an exercise like the leg press. The greater the fiber recruitment, the greater the process for potential remodeling in the muscle or, since we're referring to the Squat, this will activate many muscles. Only muscle fibers that are recruited by resistance training are subject to adaptation including hormonal adaptation and, the more muscles used in a exercise like the squat the more the muscle fibers are stimulated.
Before you begin to use weights, be sure to first execute a squat with correctly with good technique. Using one's body weight is the perfect way to start. If you can't control your own body then how do you expect to squat with additional weight?
Below is a link to my friend and 3x Olympian, Bob Kaehler on squat assessment. It is a simple test and will get you on the proper road to initiating a proper neutral load on your spine. Enjoy!