There are three main types of swimming pool filters: Sand, Cartridge, and DE (which stands for Diatomaceous Earth).
Sand Filters can filter particles from 40 to 100 microns, and are sufficient to trap most large debris in your pool's water. These filters contain between 150 - 700 pounds of special filter sand, and must be backwashed when the pressure is above the desired range. The sand in your filter can become contaminated over time by algae, debris from trees, animal hair, etc., and should be replaced every 5 - 7 years. As the integrity of the sand deteriorates, its ability to filter becomes greatly reduced and the sand bed may become "channeled" or "calcified" from hard water deposits. Gradual loss of efficiency may be hard to notice. If you are unable to keep your pool clear, even with a proper chemical balance, your sand should be changed.
Cartridge Filters can trap particles from 10 to 25 microns, and are much more efficient than sand filters. They contain 1 - 4 filter elements and do not require any backwashing. The cartridge filter element is an aquatic version of the pleated air cleaner in your car, and must be thoroughly cleaned when the pressure is above the desired range. Properly sized cartridge filters can operate for 6 - 12 months between cleanings. Each cartridge element should be replaced every 2 - 5 years, depending on the work it was asked to do. If the filter is a bit undersized, or there are significant algae and debris issues, the elements may last only 1 - 3 years.
DE Filters are the most efficient type of pool filter on the market, and can trap particles down to 3 - 5 microns; well below what the naked eye can see. They generally contain 8 filter grids, which are made of a plastic frame covered by a nylon mesh material. Like sand filters, the pressure gauge indicates a need for backwashing. After backwashing, a new application of DE powder must be added. An annual breakdown of the filter is necessary to thoroughly clean and inspect the DE filter grids. If you see sand, debris, or DE powder returning from the filter to your pool, you may have broken internal parts that need replacing. Also, a new filter may be in order if your current filter is outdated (15 - 20 years old) and difficult to use or get parts for. If the filter tank has cracked, it should be replaced immediately to prevent the tank from rupturing and causing harm to surrounding equipment, or injury to persons nearby.