The facet joints are the connections between the bones of the spine. The nerve roots pass through these joints to go from the spinal cord to the arms, legs and other parts of the body.
These joints also allow the spine to bend and twist, and they keep the back from slipping too far forward or twisting without limits. Like the knee joint, they have cartilage to allow smooth movement where the two bones meet.
When these joints get swollen because of arthritis, it causes pain.