Basically a programmer that holds your original tune so you can revert back to stock if needed. Comes with three tunes: normal, mild, and aggressive. Also can alter wheel size for speedometer and raise the speed limiter. The factory tune is so restrained and the tune simply removes the restraints and this is what the motor naturally produces without any mods other than colder plugs.
Mine is the 2.8T V6. Gets about 20 in city and 25-29 highway.
I suggest the 2.0T, it's much more economical (my old one got 38 mpg highway).
Edit: and with the tune, mpg actually increases a couple provided you're not flooring it every time.