A J-bend and a P-trap are both types of pipe bends used in Plumbing systems to prevent sewer gases from entering the building. The main difference is in their shapes: a J-bend is shaped like the letter "J" and a P-trap is shaped like the letter "P". The J-bend is typically used for sinks, while the P-trap is more commonly used for bathtubs and showers. Both serve the same purpose of trapping water to create a seal that prevents odors from coming back up the drain.