Filters remove both visible debris and microscopic matter. When your chlorine has exhausted its work, the filter steps in and performs the job of removing the now dead organisms. Periodically filters require backwashing to remove accumulated debris. Your filter manufacturer will specify the method and frequency of backwashing, although typically it is recommended when water pressure in your filter is 8 - 10 pounds above the standard operating pressure.

There are three basic types of filters. Each have advantages and disadvantages

  1. Diatomaceous earth. Also known as a DE filter. These filters have a filtration range of 1-3 microns. This is substantially smaller than any of the other filter types and is therefore generally viewed as superior. Diatomaceous earth is actually a porous powder formed form the fossilized exoskeletons of tiny diatoms ( like plankton - yuk !) . Clean water can pass through the small openings in the powder but debris and most micro organisms can not. All of these filters have internal elements that become coated with the DE 'filter cake". As with a sand filter, a DE filter must be periodically backwashed and recharged with additional DE.
  2. A sand filter is probably the most common type of filter. it has a filtration range of 20 - 40 microns. It is also the least expensive type of filter. You must use a special type of sand in these filters that is typically pool grade #20 silica sand, but check your manufacturer to match the exact specification. . Like a DE filter, periodic backwash and recharging of the filter is necessary.Typically this is done every few weeks.
  3. A cartridge filter is in the mid range of both performance and price when compared to the two previous methods. The filtration range is 10-20 microns and therefore the performance is superior to sand but inferior to DE. A cartridge filter does not require backwash. Instead you either replace the filter and/or clean the filter periodically by removing it from the housing and using a high pressure garden hose. usually this is done about once or twice  season. Cartridge filters have greater surface area that results in fewer clogs, lower maintenance and reduced operating pressure with therefore less back pressure on the pump. this leads to greater flow, higher turnover and reduced maintenance.
The eFamilyFun section of pumps and filters can be seen here.