To most of us, a pool is, more or less, a big hole in the ground. We don't see most of the expensive machinery in a pool system, because it's usually tucked away in nearby pump room. But this is the stuff that really makes the pool work.
The heart of the pool system is the water pump. In a typical pump system, an electric motor spins an impeller inside the pump housing. The impeller drives the water from the various drains through the filter and back out to the water inlets.
Just before it flows into the pump, the water passes through a metal strainer basket that catches leaves and other large debris that might clog up the pump.
Next, the water flows into the filter (or, in this setup, one of two filters). In the next section, you'll find out what happens at this point in the system.