A partial knee replacement is a procedure to resurface a damaged compartment of the knee. Traditionally the knee is though to have an inner (medial), outer (lateral), and patellofemoral (underneath the kneecap) compartments.
In some people, only one of the compartments develops arthritis. By replacing just the damaged compartment, the knee may function like there was no arthritis at all. Additionally, more bone is preserved in the event that revision surgery is needed at a later time (10-15 years later).
Recovery tends to be quicker than a total knee replacement because there is less disruption to the knee joint and surrounding muscle.