Trap (Plumbing)
Simlply put, a plumbing system supplies fresh water to a building, and removes the waste. However, all plumbing systems are connected with a swer line or septic system, creating a danger of sewer gases escaping back through the pipe into the building. Vents and drain traps exist to handle this problem.
The most common plumbing system, is the "DWV system", or Drain,Waste,Vent system. It's a network of drain pipes, they run to all the sink,showers,toilets.. etc etc. Most newer waste pipes are made from polyvinyl chloride, or acrylonitrile butadiene styrene, PVC /ABS, sealed with glue. Older systems used lead pipes or cast iron soldered with lead or other soft metals.
The soil stack is the main part of the waste drain. It's a vertical "stack" of pipes that starts in the basement/crawlspace floor (or wall) where it's connected to the sewer/septic line.The top end of the stack acts as a vent of sorts. It extends upward through the roof, allows the gases to escape outside and also helps drain flow by drawing air inward.A plugged vent can trap gases and inhibits drainage which can cause serious problems.
Make sure the vent doesn't end in the attic. Trapped sewer gases can be dangerous, (they stink) they cause serious structural problems. And a system without a vent may actually suck water out a sink's trap, or do the reverse and fill the sink with water when another fixture drains, another bad occurence.
Trap Primer
A trap primer is a valve that adds water to traps. The water in the trap is needed to prevent methane gas that rises from the sewer line from entering buildings. The water can evaporate however. The evaporation is slowed by using natural oils on the drain to barely cover the trap's "water surface". When oil and water are mixed, oil floats to the top, in effect creating a floating seal maintaining the water in the trap. It's recommended to use mineral oil, as mineral oil doesn't go rancid. The trap primer therefore is necessary to maintain the water levels.
Trap primers come in many shapes and sizes. The typical one, used for a floor drain, is a simple connection from the nearby sink. It means that when the sink is used, the water flow is diverted. Another very common trap primer is one connected to a water supply that actives when building water levels dramatically increase or decrease (ie.flushing a toilet)
Generally in western countries, building codes require trap primers for most traps, especially when there is an isolated area involves, such as a basement floor drain.