what is overflow swimming pool?

Your question can be interperited two different ways, so I'll take a crack at both.

1) There is a swimming pool overflow, which can be installed in a swimming pool to insure that the water inside won't rise to a level where it goes over the top of the decking. The reason why this would be installed is to keep water from migrating in between the structre of the pool and the concrete deck that is poured on top of it. In areas where there is expansive soil conditions, water that seeps between the pool structrue and the deck will ultimately cause the deck to crack about 18" behind the pool structure because the sloil has expanded an pushed the deck up slightly. An overflow skims excess water before it tops out of the pool and takes it away from the pool area by way of a drain line.

2) Swimming pools can be made to overflow on purpose. This is usually done on a slope where one side of the pool is higher than the other. A catch basin is constructed on the low side, and water that flows out of the pool and into the basin is pumped back into the pool. The reason people do this would be to take advantage of a particluar view - the back edge of an overflow pool is a dramatic look because the water appears to just 'end'. This type of pool is also called (among other things) a 'zero edge' or 'vanishing edge' pool. If you do a Yahoo image search pool photos using 'vanishing edge', you will be able to see many examples.

Hope that helps

http://www.freshpatents.com/Swimming-pool-overflow...

That's when you have more water than pool.