Your plumbing system is comprised of two parallel subsystems: the first brings fresh water into your house. The second, which removes wastewater from your house, is the drainage system. The latter in fact includes three components drain, waste and vent and is thus often referred to as the DWV system.
Drain lines collect water from the various fixtures; waste lines carry toilet wastes and vent pipes, which protrude from the roof, allow harmful sewer gases to evaporate, thereby also relieving pressure from the drain pipes and enabling waste to flow properly.
Originating at the various fixtures, drain and waste lines tilt downwards and are pulled down by gravity. Their final destination is the main sewer line which, in turn, carries waste matter to a sewage facility or to a septic tank.
Vents are not the only means by which hazardous gases are emitted from your house. An additional device known as trap fills a similar function. A trap is the curved section of the drainpipe, located next to each fixture. Whenever a fixture is used, some of the water emanating from it flows through the trap into the drainpipe, but a sufficient amount of water stays within the trap, and in fact forms a seal which prevents sewer gases and objectionable odors from permeating your home.