Q.
What are the common functions of Water Traps?
In plumbing, a trap is a U-, S-, or J-shaped pipe located below or within a plumbing fixture. An S-shaped trap is also known as the S-bend invented by Alexander Cummings in 1775 but became known as the U-bend following the introduction of the U-shaped trap by Thomas Crapper in 1880. The new U-bend could not jam, so, unlike the S-bend, it did not need an overflow.
Because of its shape, the trap retains a small amount of water after the fixture's use. This water in the trap creates a seal that prevents sewer gas from passing from the drain pipes back into the occupied space of the building. Essentially all plumbing fixtures including sinks, bathtubs, and toilets must be equipped with either an internal or external trap.
Sink traps have the added benefit that they trap small objects dropped in to the drain and they are fairly easy to remove. So not to worry if you drop a ring of something valuable down the drain because it is likely safe in the trap.
https://en.wikipedia.org/wiki/Trap_(plumbing)
The purpose of a drain trap. About Home, available at
http://plumbing.about.com/od/drains/a/Drain-Trap-What-Is-The-Purpose-Of-A-Drain-Trap.htm