Fusion does exactly that: two or more vertebrae are held together as one-unit. The diseased disc is removed. A titanium bone-filled cage is inserted in place of the missing disc. Bone chips are packed in and around the repair. Sometimes a plate and screws are used instead to hold the spine together.
The major problem with cervical spine fusion is the increased load it puts on the segments above the fused level. And any motion that may still be left can increase the internal stresses on the next disc above.
That's where an artificial disc replacement (ADR) comes in. The implant stabilizes the vertebra. At the same time, it still preserves motion at that segment. The thinking is that the more normal the biomechanics can be in the spine, the longer it will last without problems.
Fusion is very stable (it doesn't move at that level) but it's not mobile. Breakdown is more likely at adjacent levels. At least that's the theory surgeons are working under.
So far, only short-term results of ADRs are available. And studies comparing ADR with spinal fusion are still fairly limited at this time. Early results after one-year are very positive. Pain relief is possible with both types of surgery. Motion is definitely preserved with ADR.