Sciatica is an all-encompassing term to describe pain felt in the leg referred from a spinal source.

Technically, it implies involvement of the sciatic nerve or of its nerve roots at the spine.
When this occurs, the pain is described as lancinating, burning and severe. The patient will be able to draw a line with one finger to trace the pain. There may be true numbness or weakness in the distribution of the affected nerve.
In these cases, the pain results from irritation of the spinal nerve, most commonly from a disc protrusion. Interestingly, the pain can often be just as severe with a relatively minor disc injury as it can be with complete nerve compression.
However, there are numerous other structures that can present with leg pain. Generally, the referral pattern will be a little more vague and is unlikely to be linear. Although there may be some alterations in sensation or feelings of pins and needles, there is no demonstrable nerve dysfunction on clinical examination.
Considerations for this diagnosis include gluteal tightness, gluteal tendinopathy, facet joint related pain, sacroiliac joint pain and pain emanating directly from a lumbar disc (without nerve root involvement).
Careful examination will definitely help to differentiate these and it is important to remember that most cases of back related leg pain do not need an MRI scan. Seeing pathology on a scan does not imply causality. In fact, it is rare to see a normal spinal MRI, even in someone with no symptoms. Clinical examination holds much more value.