There are many types of systems. The most commonly installed are: Conventional System, Mound System & Aeration Unit (ATU).  A septic system is an on site sewage system and disposal system buried underground.   For purpose of discussion here we will treat them all as being the same.  The modern septic tank is a water type tank usually made of reinforce pre-cast concrete or reinforce fiberglass, through which no natural bacteria can enter.  This structure is where organic solids are decomposed with biological process.  The purpose of your septic tank is to treat household waste like body waste, laundry water, bath water, and discarded food.  Septic systems are designed to work indefinitely when they are properly installed and maintained.  It is critical to have your tank pumped (cleaned) regularly and to maintain proper levels of biological levels of activity.