Bathroom luminaire installations must comply with legislation. This has recently been updated and is now section 701 of BS7671:2008 (Wiring Regulations 17th Edition). A summary of this is printed below to assist with your choice of bathroom lightings.
This is the interior of the bath tub or shower tray.
This is the area directly above the bath tub or shower tray to a height of 2.25m above the floor. The protection required is a minimum of IPX4 (Splash proof) for normal domestic use.
This zone is no longer defined and is just "Outside Zones". Protection required is IP20 for normal domestic use.