“Rigging is the process of preparing a character (or object) for animation. It involves setting up an internal skeleton that affects where your character's joints are and how they move.”
This is what this site teach Rigging, if you want to learn rigging in Maya then this site is good for you, it has some nice tutorial about simple as well as complex rigging. You will find some useful Mel script also.
Very famous rig character Package Man is available for download with others, which actually help you to understand rigging, I also find helpful in practicing my animation skill.
