The plantar fascia is a strong flat ligament on the bottom of the foot that connects the heel bone to the ball of the foot and is responsible for supporting the arch of the foot. Plantar fasciitis is a painful conditions in which the plantar fascia becomes inflamed and has some degeneration set in.
The cause of plantar fasciitis is anything that increases the load on the arch of the foot. This include being over weight or being very active or have biomechanical problems that increase the load on the arch.
The classic symptom is first step pain that is it is much more painful under the heel when taking that first step after a period of rest (especially after a nights sleep). This is technically called post-static dyskinesia.
The best way to treat plantar fasciitis is to reduce the load on the plantar fascia by loosing weight, using arch supports (foot orthotics) and doing lots of calf muscle stretching.