Sciatica is a very common condition, especially in men and women between the ages of 30 and 50.
The pain usually starts in the lower back or hips and then radiates into the back of the thighs and can spread down into the legs with the possibility of affecting the feet. This is the path of the sciatic nerve and it's branches - it is the longest nerve in your body and runs from your spinal cord to your buttock and hip area and down the back of each leg.
Sciatica is a symptom (what you feel), not a disorder (the cause of the problem).
The radiating pain of sciatica signals another problem involving the compression or irritation of nerves as they exit the spinal canal (space which the spinal cord travels).
Hence, you will sometimes hear terms such as nerve root pain or radiculopathy instead.
Konstantinou K, Dunn KM. Sciatica: review of epidemiological studies and prevalence estimates. Spine. 2008;33:2464–2472.