First of all, what is a water pump and why should I care? Water pumps are a critical component (among many others) that circulate engine coolant/antifreeze around the engine and out to the radiator. When they’re not working your vehicle will be overheating and you should stop the car immediately to avoid any permanent damage. Like any mechanical device, they eventually wear out.
Some water pumps are driven by the serpentine belt and are bolted on out in the open with the alternator and air compressor and such. Other water pumps are driven by the timing belt. Water pumps that are driven by the timing belt (the belt that 
controls when your engine intake and exhaust valves open) are attached directly to the engine block. The timing belt system has protective cover. The serpentine belt and all the accessories are located over the timing belt cover. To make a long story short, getting to the water pump requires the removal of a lot of parts – which then need to be replaced.
So when the water pump must be replaced, it is a good idea to go ahead and also replace the timing belt, timing belt tensioner and idler pulleys. First, because the timing belt may have been contaminated by leaking coolant; second, there is a recommended manufacturer schedule for replacing the timing belt and at the end of the day you’ll be saving the labor cost because the technician will pretty much remove the same parts when doing this later.
Finally the reverse is true as well: When you are replacing your timing belt you should also d replace the water pump at the same time for the same reason. Do them at the same time and start the clock over for both.