Pex is short for polyethylene, cross linked. It's a very durable and cost effective pipe, and is what we recommend most when replacing the water lines in a home. Pex has better resistance to freezing than copper, doesn't corrode, and has lower material and labor costs than copper.
Pex pipe that we put in because the cpvc ball valve had failed. With pex we use brass fittings and valves and copper crimp rings.