A trapped nerve in your back is usually caused by a prolapsed or slipped disc, which then narrows the spinal canal where the nerves pass through. You can also get a trapped nerve from scarring form previous surgery.
The pain then runs along the site that the nerve travels down.
Sciatica pain comes from the lumbar-sacral spine which is at the bottom of your back and shoots down your thigh, knee or foot on the same side of your body. You can get pins and needles or even lose sensation around the area and the muscles can become weaker.
At the moment due to a knock I am suffering from nerve root pain (I think) as I have pins and needles at the base of my neck and my left arm feels very weak.
You should always get this sort of pain and discomfort looked at pretty quickly to avoid any permenant nerve damage (note to self).