Whether you have a preformed or a liner pond, a pond pump is a paramount component of any water feature. Stagnant bodies of water attract mosquito infestations, which are a nuisance, and also undesirable, due to the recent outbreaks of the West Nile virus. Therefore, it is paramount that you utilize a pond pump that circulates your body of water.
Pumps are available in both submersible and external (out-of-pond) models. For the smaller pond (up to 1000 gallons of water), a submersible pump is the more economical and practical option. Submersible pumps can be placed directly in the pond and require relatively little installation. They are free of distracting noise, and for smaller ponds, can easily be utilized to drain your pond (if you hook up a hose to the output flow).
In the old days, the main disadvantage of submersible pumps was that the pump seal could rupture and release oil coolant into the water. However, this is not the case nowadays, because newer pumps are magnetic-driven, and no longer require coolant.