To answer the question, 'what is sciatica' it is helpful to explain some of the relevant anatomical structures. The sciatic nerve is both the longest and the widest nerve in your body. It begins in the lower back as five separate nerves that extend from five different vertebrae - L4, L5, S1, S2, and S3. These nerves meet up to form one large bundle which runs through the buttocks and down the entire length of the leg. The sciatica nerve is responsible (directly or indirectly) for nearly all the sensation in your leg including the skin of the thigh and gluts.
Sciatica occurs when there is compression or irritation of the sciatic nerve. The pain is felt in the low back and leg, but the site of the pain isn't always the site of the underlying issue. Since the sciatica nerve is so long and travels around or through the large muscles of the buttocks, it takes an expert to determine if the pain is coming from disc compression, from muscle spasms or from something else. Again, sciatica is a symptom, not a diagnosis.