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Disc herniations can be grouped or classified based on location and type of protrusion. Locations include central, posterolateral, foraminal, and lateral. Central means the disc pushes straight back equally on both sides. Patients with central protrusion have as much back pain as they do leg pain.
Posterolateral herniation means the disc has pushed back but at an angle to one side or the other (left or right). Disc herniation straight out to the side is in the lateral group. The foraminal zone is between posterolateral and lateral. Foraminal refers to the opening for the spinal nerve to exit as it leaves the spinal cord or spinal canal area. Leg pain may be more pronounced with these three types of herniation.
All of those classifications are based on location. But discs can be classified according to the type and extent of protrusion. The terms protrusion and herniation are often used interchangeably. But technically, protrusion is a type of herniation.
For example, disc material that pushes past its boundaries equally with a broad base and maintains that width in all planes is a herniation that is a protrusion.
A disc herniation that pushes past its covering and then bulges wider than the base is another type of herniation called extrusion. Sequestration occurs when the disc material breaks off and becomes a free-floating fragment inside the spinal canal.
So in your case, the nucleus (inner portion) of the disc has pushed into the outer part or covering of the disc (annulus). It has moved to the back and toward one side. On an MRI, it would be a bulge like a bubble -- rounder or wider at the end of the protrusion than at the base.
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