How does a below knee leg differs from the above knee leg?

A below the knee prosthesis means the person still has their knee cap(or knee joint) and the prosthesis attaches below there.
An above the knee prosthesis includes, as part of the prosthesis, a bendable knee joint built in. The prosthesis would attach usually somewhere on the femur (thigh bone).

I am a 22 year old male with a prosthetic leg. I lost my leg when I was 15 by getting hit by a truck. I am an above the knee amputee so to answer your question, like the other person said above the knee means you are an amputee with no knee joint below the knee you do. Above the knee takes longer rehab time because you have to learn how to walk basically on a stilt that bends. Learn to keep your balance. Below the knee takes less recovery and rehab. Below the knee since they still have a knee joint it is easier to walk because you don't have to learn how to walk while bending an artificial knee. I hope this helped.