A septic system, also known as an on-site sewage system (OSS), stores, treats, and disposes of the things you flush and send down drains. There are many different types of systems. Some are simple, using only a tank and a drainfield, and others are more complex, requiring pumps, filters, or specially designed materials. The term on-site sewage system is used in the state and county code, on permit application forms, and by industry professionals to refer to different types of septic systems.