What is the main difference between hip resurfacing and total hip replacement?

Placement of a stem: The "main difference" is that with a total hip arthroplasty a cut of the femur is made (osteotomy) and a stem is placed inside the femoral canal. The hip ball is connected to this stem. With hip resurfacing the diseased femoral head (ball) is capped with a metal implant and the femoral neck is not cut.

The femoral implant: In hip replacement the part in the tight bone (femoral) fits inside the shaft of the bone. In resurfacing, it covers the ball (femoral head), preserving more bone.

Femur and bearing: A hip resurfacing and total hip both involve relining the inside of the socket. A resurfacing does not remove the entire head, but rather caps the head with a cemented metal piece. A total hip does replace the entire head, and the head is attached to a piece of metal that goes down inside the femur 5 - 6". A critical difference is that a resurfacing requires a metal on-metal bearing (bad idea).