Precast concrete is a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and maneuvered into place; examples include precast beams, and wall panels, floors, roofs, and piles.
Precast concrete is reinforced concrete that is cast away from the building site, and assembled on site. Some (but not all) precast concrete is available in standard shapes and dimensions: floor and roof planks, tees and double-tees are examples.