Buttons send a redstone pulse when right clicked. This means that they will do things (when placed in the right spots) like activate pistons, open doors, start minecarts, and lots of other things. The variation in buttons is not just for looks, it also varies the length of the pulse sent out. Buttons are also in vanilla minecraft, so you can make them out of stone and vanilla wood.
For more about redstone and its potentials, go to this forum page.