A conventional septic system has two main parts. The tank holds wastewater long enough for solids to settle out. The drainfield is a set of lines in the ground where the liquid soaks into the soil and gets treated. Some sites call for a different design, such as an aerobic system, depending on the soil and the lot.
Putting one in takes planning before anyone digs. The soil has to be evaluated, and the system has to be sized for the site and the household. In Texas, septic work follows TCEQ rules, with permits issued by the local authorized agent (usually the county), or by TCEQ where there isn't one.