What is P or S trap

P trap
http://www.rensup.com/t/P_Traps_Dtl99071.jpg

S trap
http://www.signaturehardware.com/images/plumbing/2/sh-s125c_d.jpg

In both cases waste water drains from above and flows through the trap line to the waste line... notice that some water will always stay trapped in bottom of the U shape... this makes a water seal that prevents odors and noxious gases from the waste-vent system from entering the home.

Think about a portapotty... stinks, eh? No trap. The traps you mentioned help keep those nasty and dangerous gases from entering your home.

A sink's P-trap is there to seal off the pipe with water to keep sewer gas that bypasses the vent from rising into your bathroom

A P trap goes through the wall an S trap goes through the floor.
S trap can self siphon and thus it is against most codes in civilized countries.
Most codes frown upon carcinogenic fumes from entering a structure.

I hope the above info will help you, Thanks.

John