Key poses are the "key" moments that most clearly describe the important physical actions or emotional moments in a scene. These are basically your most important poses -- in fact, these are so essential to the movement/acting that if you removed any one of them from the scene, it would no longer work.
For me, I use a LOT of key poses. I find it helpful to be very detailed with my key poses. In a full-figure animation, a key, for me, will be any moment something important changes. It could be that the character is going to take a step to the right, so he has to shift his weight to the left first. Even though it isn't a "big" action, I will think of the weight shifting to the left as a "key pose" or an "extreme," which are both terms that are often used to describe the same exact thing, depending on who you are talking to. So, I will save a key on every controller on the frame before he starts to shift his weight (this is a key pose for me), then I'll go to where his weight shift finishes, and I'll pull his body to the left some. Then, even though I've only moved a couple controllers, *I'll save a key again on EVERY controller on the character,* including hands, shoulders, etc. This is another key pose or extreme, for me. I save keys on everything because it makes it much easier to edit later on, and much less confusing in the graph editor when you are blocking. Eventually, I will have to break up my keys somewhat as I get into fine-tuning the animation and polishing it up, but for now, it's good to work this way.
An easier example to think about might be a bouncing ball animation, and the key poses would simply be the frames where the ball hits the ground, and then the frames where the ball is at its highest peak.
- Shawn :)