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The trap under your sinks and under your tub and shower.

I get many questions about the 'U' or J shaped fitting under the sink.
It is called a trap, and what it does is it traps water in the pipe. why does it trap water in the pipe? the water in the trap prevents sewer gasses and odors from coming into the house from the septic system or the local sewer system, some traps go in to a pipe inside a wall that has a vent that goes through the roof this type of trap is called a P Trap this type hook up is preferred and is in line with the plumbing code.
another type of trap goes through the floor, this is called a S Trap this type of trap does not have a vent and is not up to plumbing code. A S trap can cause the water that is meant to stay in the trap to siphon out allowing sewer gasses to come into the house, this can be dangerous and smell very bad. sewer gasses contain methane gas that can be flammable and unsafe to breath.

Many people think a plumbing vent is to vent the sewer odor from the house, but that is not it's function, a vent pipe brings air into the drainage system to allow the water to drain, next time you have a glass of something that you use a straw to drink, if you hold your finger over the top of the straw, note how whatever is in the straw will stay in the straw even if you lift it all the way out of the glass, but as soon as you remove your finger from the top of the straw the liquid drains right out of the straw, and that is what the drain vents do in your drainage and vent system.

Toilets are a S trap themselves, and you can see the siphon action take place when the toilet flushes, but the toilet is designed to refill the bowl after every flush, keeping the trap full of water, the toilet also has a vent that goes through the roof, a toilet can not flush without a vent.
Some homes have a main trap or a house trap that connects to the main sewer line just before it goes out of the house. local coded determine the need for a house trap or not.
although you may not be able to see the traps under the bathtub and showers they are there.