This interactive video gives a clear picture of degenerative spondylolisthesis [1] and isthmic spondylolisthesis [2], which can cause low back pain [3] and/or leg pain as the vertebrae slip forward.
Links:
[1] http://www.spine-health.com/Conditions/Back-Pain/Common-Causes-Of-Back-Pain/Lumbar-Causes/Degenerative-Spondylolisthesis.html
[2] http://www.spine-health.com/Conditions/Back-Pain/Common-Causes-Of-Back-Pain/Lumbar-Causes/Isthmic-Spondylolisthesis.html
[3] http://www.spine-health.com/Lower-Back-Pain.html