There is rigid and flexible plastic and metal. It depends on specific applications and local codes. Plastic is less expensive and easier to work with. Rigid can be more difficult to place but is stronger and is easier to pull wire through. Use the gray PVC for electrical rather than the white for plumbing. Metal conduit is an outstanding shield if high and low voltage must run close together and one type is in metal conduit. Never put high and low voltage in the same conduit but the lower voltage stuff like speaker wire and interconnects coexist nicely.