Credit: Original article published here.

Precast concrete is simply concrete that is not cast in its final position, hence, pre-cast. It is cast in a reusable mold and is typically cured in a controlled environment. The last step is moving it to its final, or in-service, position. Cast-in-place concrete is simply concrete that is cast in its final position. So, while both are made of concrete, and contain rebar and steel, they are a bit different in what they can get us, and how we get to the final product. Figure 1 shows precast being swung into position while Figure 2 shows poured-in-place concrete being pumped into position.