A. in office is where you go to the dentist and just have the teeth lightened under a special light/laser/power bleaching ,although quick this result fades if not combined with home whitening . zoom and many other companies do power and home whitening (which is a system that uses special custom mouthguards that you wear for about 40 min a day for approx 7-10 days). for best results the two have to be combined . usually when you see advertisements for whitening that is cheap they are doing one or the other, but studies show the combination is much more effective and hence the higher cost