A p-trap and a J-bend are both types of pipe fittings used in Plumbing systems to prevent sewer gases from entering a building. The main difference between them is their shape: a p-trap is shaped like the letter "P" and a J-bend is shaped like the letter "J". Additionally, p-traps are typically used under sinks and bathtubs, while J-bends are commonly used in toilets. Both fittings serve the same purpose of trapping water to create a seal that prevents odors from escaping.