The cruise altitude is programmed into the FMGC during initialisation. The pressurisation computer then looks at the cruise altitude and the rate of climb immediately after take off and calculates the climb pressure differential to give a nice smooth cabin pressure rise during the flight whilst taking into account step climbs.
The FMGC also contains a database of company destinations which, when the pressure mode selector on the overhead panel is set to auto, it uses to determine the height of the destination so that it can reverse the pressurisation and ensure that the outflow valve is fully open upon touch down.
Thus it gives a nice, gradual pressure change that isn't too nasty on the ears. Especially good for the kiddies!
In the little window in the passenger doors are two lights, a white one for slide armed warning and a red one for excess pressurisation in the cabin on the ground. At that point you would take the pressurisation system into manual and motor the outflow valve fully open.
Hope that answers your question.