I've read a lot of different books about bodyweight exercises. One of the things that has left me a little unsatisfied however is definitions of the various bodyweight exercises. Some are  lacking whereas others are at best incomplete. This is why I decided to create this list of definitions below:

Calisthenics

I was actually having a hard time finding a good definition of calisthenics until I went to good old Wikipedia. This is where I found this definition:

Calisthenics are a form of organized exercise consisting of a variety of simple movements performed without weights or equipment that are intended to increase body strength and flexibility using one's own body for resistance.

Does this mean what Richard Simmons does is calisthenics? I don't think so. Just jumping around doesn't count. For an exercise to be considered calisthenic it must combine some form of your bodies gravitational pull and leverage to supply resistance to your muscles. In mind exercises like pushups as well as animal exercises like bear crawls count as calisthenics.

Self Resistance or Isotonic Exercises

These exercises use an opposing body part to supply resistance to the muscles rather that using a physical weight. For example consider an exercise like the biceps curl. Instead of curling a dumbbell you use your opposite hand to resist the curling motion. Your muscle fibers respond to resistance. It doesn't matter whether it's an actual weight or something else.

Visualized Resistance

This one is hard to believe, but it works. Almost any muscle in your body has an opposite, opposing muscle. For example your arm contains the biceps, which will bend the arm at the elbow, and the triceps, which will straighten it. The idea here is that if you want to work your biceps you will perform a curling motion. However you will also use your mind to tense your triceps muscles at the same time.

Like I said before your muscle fibers respond to resistance no matter what kind of resistance it is. Another reason this can be so effective is because it forces you to really focus your mind. Always remember it is your MIND that works your muscles. When you are focused on the task at hand and not letting your mind wander your workouts will be twice as effective. This goes for any kind of exercise that you can think of.

Isometrics

The word "Isometric" comes from, where else, the Greek. "Iso" meaning equal and "metric" meaning length or distance. Isometrics are any exercise in which tension is applied to the muscle but there is no actual movement. One way to practice it is to use self-resistance with no movement. Another is to work your muscles against some object, again with no movement.