I'm sure HC will correct(flame) me if I'm wrong or forgot something, but the Controls.dat project will provide information that frontends will be able to use to display extended information about a game in Mame. Basically, this information is:
-Number of simaltanoeus players
-What do the buttons in each game do? (p1b1 = fire,jump,shoot,etc..)
-What controls are user for each game (4-way, 8-way, analog controller, spinner, trackball,etc..)
-other stuff I'm sure I'm forgetting...
What this will do for you:
Now, theoretically (and hopefully soon - practically ), you can design what your control panel looks like from your cabinet (joysticks,buttons, etc) using a layout editor, and when you use a front end that can read the controls.dat, you will be able to see how to play each game using your control panel, and what controls on YOUR control panel map to the ORIGINAL games' controls.
Hope this helps.
-PMF