A Model Sheet (in the animation industry) is basically a bunch of drawings of a character, in a variety of poses and expressions that is used as reference material by other animators during the production process.
Examples:

johnkstuff.blogspot.com

animationarchive.org

In Flash, I use a method that I call "Live Model Sheets". Instead of a static drawing, I keep all the pieces and the actions of the character in a single flash file. It's Kind of part Model Sheet, part Model Library.

This is a lot more work in the beginning but pays off big when you're knee deep in the production process. By having all the assets in a single file all drawn the same size it is easier to swap and replace elements that make up a character. If you are building a flash animation/s with a recurring character this is by far the best way to go.