The "Show Some Backbone" tutorial shows how to scale the premade rig to fit a character. For most characters it will work well and you don't need know about most constraints. Follw the tut and scale it to match your character instead of Thom.
But it doesn't cover constraints. If you want to know more about how the constraints work, here's a walk thru on the simplest possible IK leg
Simplest_IK_Leg_H.mov
Once you digest that there's a written tut on a more functional IK leg setup
David Rogers' Tut