In Waterfall model each step is distinct.After completion 
of step one can move to next step and once you are through 
from one step you cant move back.Hence waterfall model is 
useful only those projects where requirements are well 
understood and no chang in requirements is needed.

In spiral model we dont define every thing in detail at 
very beginning.we start with basic functionality and work 
them out.we take client's feedback.If developed 
functionalities is as per clients requirement then we move 
on to next step.Spiral model is useful when requirements 
are changing.