The Spiral Model combines the features of the prototyping model and the waterfall model. It produces a working system using the waterfall model and after its release it then reverts back to the first phase and heads down the waterfall over and over until the system is sufficiently refined. This allows for incremental releases of a product, or incremental refinement through each time around the spiral.
The advantage of the spiral model is that it can be used with very large, complicated, expensive and continually changing systems.
However, it can often be too complicated or demanding for smaller projects.