The sewer mains must be installed deep underground to provide service to users and avoid other utilities like water lines. Sewers also need to slope continuously downhill for continuous flow of the sewage. At some point the pipes are so deep, or there is an obstacle like a river, that it's not practicable for the pipe to continue. At this point a sewage lift station is constructed that serves the basin of users located upstream from the lift station.
A lift station may have many configurations but generally they have a deep well (perhaps 20 to 30 feet deep) where sewage collects. The depth of sewage is monitored and at a certain point a switch is triggered and pumps driven by electric motors kick on and the well is pumped out. This cycle continues and, for example, is more frequent in the morning when residents are showering and less frequent late at night when residents asleep.
A lift station always has at least two pumps for redundancy and extreme high flow conditions when both pumps are needed. A two-pump lift station is called "duplex". A three-pump lift stain is called "triplex". The pumps and motors can be located above ground, below ground in a "dry well", or submerged directly in the sewage. A submersible pump is a sealed unit that has both the pump and motor.