It is an irritation of the sciatic nerve, which is the largest peripheral nerve in the body.
It's important to differentiate sciatica from other lumbar radiculopathies (shooting pains coming from the lower back).
While it is possible to see shooting pains going into the front or sides of the legs, true sciatica is only irritation of the sciatic nerve, and therefore the pain or burning sensations will follow the course of that nerve, which is down the center of the back of the leg.
Misdiagnosed shooting pains are a problem because they lead to incorrect treatment, obviously.
That's why it's so important to recognize which nerve is being affected, so the cause of the problem can be more accurately identified and then addressed with proper treatment.
In the case of sciatica, the cause is typically compression of the nerve in one of two areas: either where the nerves first come off of the spinal cord and emerge from the spinal column (through little holes called the IVF), or from compression by the piriformis muscle, which is a muscle deep in the pelvis/glute region.
That second cause is so common that there's even a term for it: piriformis syndrome.
The sciatic nerve travels directly underneath this stabilizing muscle (actually, the nerve even goes directly through it in about 10% of people), and a spastic or lax piriformis can put pressure on the nerve, causing the classic shooting pain symptoms.
Problems at the nerve root, or where the nerves come off of the spinal cord and past the vertebrae to go out into the body, are usually disc-related.
When you suffer from a bulging disc, sometimes the bulge happens to be right where the nerve is trying to pass through those small IVF holes through the spinal column and out into the body.
When this happens, the bulging disc presses against the nerve root and causes shooting pain or other radiating symptoms.
This doesn't just happen to the sciatic nerve; it can happen to any nerve in the body.
It's just that when the sciatic nerve is specifically affected, it's called sciatica.
Treatment for sciatic nerve pain depends on the cause.
Piriformis syndrome is usually handled with certain stretching and range of motion style therapies, while disc bulges are handled by traction, spinal decompression, or in very bad cases, surgery.
Your doctor can tell you more about treatment options.