A P-trap has a horizontal pipe exiting through the wall while an S-trap curves downward into the floor. P-traps are preferred in modern plumbing codes because they maintain a proper water seal preventing sewer gases from entering your home. S-traps can lose their water seal through siphoning which allows harmful gases to escape. Most building codes now require P-traps for new installations. If you have S-traps consider upgrading for safety and code compliance.