Spondylosis – general term for degeneration in the spine. Includes both degenerative disc disease and degenerative joint disease (facet joint arthritis)
Spondylolysis – defect is the pars interarticularis. Not related to spondylosis even though the words sound similar.
Spondylolisthesis – anterior slippage of one vertebral body relative to the next. Can be caused by either spondylolysis or arthritic changes in the facet joints.