As discussed on the bathtub maintenance page, there are a variety of different tub spout diverters, but they generally fall into two groups: 1) full shutoff valve; and 2) pull knob on the bath spout.
The full shutoff valve model is preferable because it eliminates water waste. If you are remodeling your bathroom you should specify the full shutoff (three valve) model. If you must buy a pull knob model, insist on a product guaranteed to leak 0.5 gpm or less. Some diverter valves are designed not to leak at all and these are certainly preferable from a water conservation standpoint.
Full shutoff valve with three valves
Photo courtesy of Price Pfister
When replacing an old diverter valve, be sure to bring the old parts to the store with you to ensure that you purchase a diverter valve that fits your fixture. It is also important to know the brand of fixture you are repairing.