P-Trap or S-Trap, What's the Difference?
Firstly, a trap is the bend in a pipe designed to prevent sewer gases from entering buildings.
"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." - Source: Wikipedia
A toilet will always have one of the following two types of traps integrated into the pan.
The position of the main stack or sewage outlet in the building determines whether the required shape trap, and thus the style of toilet needed.
A P-Trap exits at the back of the toilet and is designed to outlet through a wall, or into a 90 degree bend through the floor or off to the side.
An S-Trap is designed to outlet directly through the floor, and thus the poistion of the stack or sewage pipe in the building must be checked piror to selecting a toilet.