Zone 0 and Zone 1 Regulations
Zone 0 is inside the bath or shower itself. Any fitting used in this zone must be low voltage (max 12 volts) and be rated at least IP67 which is total immersion proof.

Zone 1 is the area above the bath to a height of 2.25m from the floor. In this zone a minimum rating of IP44 is required. If the fitting is 240 volts, a 30mA residual current device (RCD) must also be used to protect the circuit in this zone.

Zone 2 and Zone 3 Regulations
Zone 2 is an area stretching 0.6m outside the perimeter of the bath and to a height of 2.25m from the floor. In this zone an IP rating of at least IP44 is required. In addition, it is good practice to consider the area around a wash basin (within a 60cm radius of any tap) as Zone 2.

Zone 3 is anywhere outside zones 0, 1 and 2 (subject to specific limits) and where no water jet is likely to be used. No IP rating is required in Zone 3. In addition to the above, if there is a likelihood of water jets being used for cleaning purposes in zones 1, 2 or 3, a fiting rated a minimum of IP65 must be used.

Full details can be found in the latest copy of the IEE wiring regulations.