Each spiral can be termed as a loop and each loop is a separate development process in a spiral model. The four activities (Planning, Risk analysis, engineering and evaluation) form the intermediary phases of a spiral model and is repeated again for each loop.
This model is very good to use for larger projects where you can develop and deliver smaller prototypes and can enhance it to make the larger software. The implementation of this model requires experienced resources as risk analysis is a very integral part of this model and risk analysis requires expertise and as a result this model becomes costly.