Standard septic tank systems consist of two parts: an underground tank and a set of perforated pipes in a leach field.
The septic tank itself is usually made of fiberglass or concrete, and may hold 1000 to 2000 gallons (4000 to 8000 liters). Household wastes flow into the tank, where anaerobic bacteria break them down into simpler chemical compounds. Heavier solids drop to the bottom of the tank as sludge, lighter solids float to the surface as scum. Everything else flows out through a pipe into the perforated pipes in the leach field, an area frequently filled with 1-inch (25 mm) gravel. The liquid flows out through the perforations into the gravel area, and from there out into the un-disturbed surrounding soil.
Every few years you need to have the septic tank "pumped," which removes all the scum, liquid and sludge. If you delay pumping the septic tank too long, the sludge level may build up so much that sludge particles flow out into the leach field and clog the pores of the undisturbed soil. When this happens the soil can no longer carry liquid away, and you begin to experience moist ground, unpleasant odors and drains in the house that no longer work.
In many areas of the United States pumping a septic tank will cost $100 to $300. Replacement of a leach field is likely to cost $10,000 or more, with wide variations due to differing local regulations.
You will find many more articles that tell you how to live with your septic tank here.