In this type of pools, water flows to the filtration system through skimmers located at the top, and the main drains at the bottom. All the debris that floats on the surface (leaves, hair, etc.) is drawn from the skimmers, whereas most of the dirt that sinks exits the pool through the main drains. Once the water is filtered, it returns to the pool through the wall inlets. The number of skimmers is determined by the area of the pool. In general, there are several main drains and skimmers; therefore if something blocks one of the drains, the pumping system pulls water from one of the other drains.
Swimming pools with a skimmer are usually not big, private pools. This type is relatively cheap and easy to build. Filtration type here is horizontal. The water line is up to the level of the skimmers and through its holes water is overflowing all the time. After being treated, it’s flowing back through nozzles, mounted most often on the wall opposite of the skimmer.