These are the more common joint surface problems in the knee:
Cartilage softening & destruction
Cartilage (the 'gristle') softening is called 'chondromalacia'. This may progress to cracks, blisters and breakdown of the cartilage.
When cartilage destruction on the inner aspect of the knee joint progresses to involve the underlying bone leading to pain (especially gnawing at night), swelling of the joint, outward thrusting of the joint while walking, and bow-leggedness.
Lateral compartment arthritis
The same but the destruction is on the outer side of the joint, and the tendency is for the knee to become knock-kneed.
A blow on the knee or a fall may bruise the bone and fracture the cartilage.
A blow on the knee might dislodge a chunk of cartilage and underlying bone, leaving a crater. The free bit may float around in the joint cavity as a 'loose body'.