in flash, buttons have 4 states, in the form of 4 frames
inside a symbol.
1) Normal (up)
2) Over (on RollOver)
3) Down (onPress)
4) Hit (hit area)
Each of these can be changed as its own keyframe. You can
also control what you're talking about with actionscript, swapping
movieclips around the stage. Macromedia has a bunch of little
tutorials that would cover the button stuffs bundled with your
original Studio package.