The guys over at AMDmb.com have posted a Water Cooling Equipment and Performance Guide dealing namely with pumps. Written by Amdmb.com's Dave Smith, an experienced electrical engineer, the article details everything you would need to know about pumps, pump ratings and efficiency and flow.

The pump flow rating is probably the least understood concept. Much as I explained the flow ratings of fans and how you'll never get the rated flow, the same holds true for most pumps. Almost without exception, the pumps used in water cooling are centrifugal. To explain what this means, think of a merry-go-round. Water enters at the center of the pump impeller. Because the impeller is spinning, the water wants to fling outward from the center, much like riding on a merry-go-round. The discharge port is located at the outside of the impeller and receives the water from the impeller.