Spondylolisthesis is the shifting of one vertebra forward on another vertebra. This causes many problems at once:
1. The spinal canal does not line up correctly and it is "kinked" thus creating nerve irritation and/or nerve damage. 
2. The joints of the vertebra do not line up correctly these become quickly painful and arthritic, leading to pain.
3. the nerves that come out of the spine become constricted due to the hole in the bone above and below the vertebra kinking.
There are 2 different types of spondylolisthesis:

1. Degenerative Spondylolisthesis

2. Spondylolysis also called Lytic Spondylolithesis