If you are diagnosed with a protruding disc it means that the gel is pushed out or bulging, and is asymmetrical in different places. This may result in pinching and irritation of the nerve roots at that point.
A herniated disc happens when the disc material ruptures to the point that the gel pushes outward. This can also cause pain and numbness due to the irritation of the nerves and can be very serious in some cases.
A disc extrusion is the worst type of disc injury. What it means is that the gel balloons extensively into the area outside the vertebrae or breaks off from the disc. This tends to result in considerable back pain and limitation of range of motion.