Along with proper chemical balance, maintaining a filter is vital for a clean, swim-able pool. If you have a pool more than 10 years old, you should consider replacing the entire filter. In decades past, the filters put in pools are almost universally too small by today's standards. There are 3 major types of filters used today and you can learn more about them here.
The first is called a Diatomaceous Earth filter. This system utilizes a substance known as DE (fossilized skeletons of micro organisms) and will trap the tiniest molecules of dirt or algae (as tiny as 5 microns). As the filter does its job, the pressure rises, and it is generally the rule with all types of filters to clean the filter at 9PSI over when it is running clean. When the pressure rises that high, back flush and clean. After the filter is cleaned thoroughly, simply add another helping of DE powder and you are set.
The second widely used type of filter is a sand filter. A special type of sand is used to trap particles in the water (usually between 20 and 100 microns at first) and as more particles are trapped, the filter gets tighter and thus, more is trapped, until it reaches the commonly held rule of 9 PSI above a clean filter, in which case you back flush the dirt out.
The last major filter type is called a Cartridge filter, and is mostly used for smaller bodies, such as spas and above ground pools. A properly sized and maintained cartridge will last between 3 and 5 years. The cartridge is loaded in and collects particles at about the same level as a sand filter. When the cartridge is pulling at a high pressure (again, about 9PSI above normal) simply remove it, spray it down thoroughly with a high pressure hose, then replace it. If you have more questions on which type of water filter you feel is best, read our guide at openwaterhq.com.