remapping is a form of engine tuning. certain cars come limited as standard..remapping a car will unblock the restrictions. depending on what you want, you can change the way the car is mapped to suit your needs..eg - Increased throttle response, Flat spots will be ironed out, smoother power delivery, quicker acceleration, and so on..
Tuning companies usually rolling road the car to see what your car is performing at , and to find out what bhp your car is they print out a dyno graph. you get a lot of people claiming that their car is 1000bhp or some stupid unrealistic figure but until theres proof of a dyno graph consider them talking shit! thers quite a bit of banter on passionford in regards to people claiming their car is faster than supercars but nevermind hey!
when your running a turbocharged engine you should get it mapped around once a year. just to make sure everything is running sweet.
you can also get you car live mapped. have a look at stu's link from msd
http://www.motorsport-developments.co.uk/whatismap.html
theres loads of info out there just read mate. its interesting stuff.
jano from oddkidd mapped my escort, and he done a bloody good job of it!