Anatomy of the Spine

The normal anatomy of the spine is usually described by dividing up the spine into 3 major sections: the cervical, the thoracic, and the lumbar spine. (Below the lumbar spine is a bone called the sacrum, which is part of the pelvis). Each section is made up of individual bones called vertebrae. There are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae.

An individual vertebra is made up of several parts. The body of the vertebra is the primary area of weight bearing and provides a resting place for the fibrous discs which separate each of the vertebrae. The lamina covers the spinal canal, the large hole in the center of the vertebra through which the spinal nerves pass. The spinous process is the bone you can feel when running your hands down your back. The paired transverse processes are oriented 90 degrees to the spinous process and provide attachment for back muscles.
There are four facet joints associated with each vertebra.

A pair that face upward and another pair that face downward.
These interlock with the adjacent vertebrae and provide stability to the spine.
The vertebrae are separated by intervertebral discs which act as cushions between the bones.
Each disc is made up of two parts. The hard, tough outer layer called the annulus surrounds a mushy, moist center termed the nucleus.

The Neural System:
The spinal cord  is based in a tube called the spinal canal, and is enclosed in the vertebral bodies, for protection.
Between 2 vertebra is a space called the neural foramen. At each level 2 nerves leave the spinal cord , through the neural foramen and go on to supply the muscles , skin and bones.
As the nerve leaves the spinal canal, it is closely placed behind the disc. Problems with the disc can cause the nerves to be affected, resulting in pain/weakness/loss of sensation in the nerve pathway. This can vary from a torn piece of disc pressing the nerve( Disc prolapse), to a narrowing of the spinal canal causing nerve irritation( spinal stenosis). Disc degeneration can result in some instability and this can cause pain down the nerve, even without compression of the nerve.
Other diseases can cause nerve injury- like diabetes, or multiple sclerosis.
Pressure on the spinal cord higher up( say in the neck) can cause symptoms of  heaviness in the legs, pins and needles in the hands , loss of balance, bladder problems and leg weakness- called cervical myelopathy.