In a total knee replacement (TKR) the end of the long bones of the knee joint (thighbone or femur and shinbone or tibia) are sawn off with special jigs, and the bits replaced with artifical materials.
The femur bit is usually replaced with metal (eg. titanium or zirconium), while the tibia bit is usually plastic or metal and plastic. The procedure is also often referred to as an 'arthroplasty'.
The important thing to remember is that there is no going back after this procedure. The removed bits of bone are thrown away. Revising (re-doing) the procedure is complicated - so the best chance is your first chance and so it is worth finding the best surgeon in your area.
The surgeon uses a tailored jig to be sure where to make the bone cuts. The prosthesis may be cemented onto the bone, or may rely on special coatings to fuse to the bone.
Early on there will be swelling and bruising. Swelling may last several weeks. The knee will feel different. will be initial stiffness and weakness of the muscles.