PEX (cross-linked polyethylene) fittings are a type of fittings used in plumbing systems that use PEX pipes. PEX pipes are flexible and durable, making them suitable for a wide range of plumbing applications. PEX fittings come in different shapes and sizes, such as elbows, tees, couplings, adapters, and valves, and are available in various types of connections, such as expansion, crimp, clamp, or push-fit.