The waterfall model the process goes to the next step after 
completion of the previous step as first requirement then 
design then coding then implementation then maintenance but 
here is no end user feedback taken to consideration any 
change in SRS will result to start work fro first step and 
goes step by step again.
But in case of Spiral model for each and every step there 
is testing for that step carry on simultaneously after 
finishing that step so that it will easy to recover any 
error and fix it there. In this model we don?t have to 
start work from beginning