All of your sewage usually go through the main sewerage system. A septic tank will provide you with a sewage system separate to this.
There are many businesses and households that are in the unfortunate position where they do not have a main connection to the public sewage system.
Common soakaways are constructed via long trench lines filled with a clean stone and have a slotted or holed distribution pipe.
Cesspits are completely sealed tanks which collect all the effulent. They require frequent emptying due to not having any drainage facility.