What is water-sealed latrine?

A water sealed toilet system, or latrine or kitchen sink, is one where there is an S or P bend beneath the appliance such that there is a water barrier between the appliance and the sewers. This is necessary to prevent sewer gases from entering the building.

In addition to the water seal, there should also be a vent to air on the sewer side of the appliance, to ensure that pressure fluctuations in the sewer system will not destroy the water seals. (or suck them away).