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

In the water fall model the process flows from top to 
bottom like a flow of water . But any new changes can not 
be incorporated in the middle of the project development.

Whereas the spiral model is best suted for projects 
associated with risks.

water fall model is casecade in nature, because it goes 
from one stage to the other and it cannot come back to the 
prevoius stage. it follows the principal like
Requirement gathering
Analysis of the requirements
Design
Coding
Maintenance
while in spiral model the concept of waterfall model is 
combined with the iterations. it is also different in the 
design from the waterfall model, after completing one stage 
we can easily come back to the pravious stage.

Waterfall model is a document driven process.

While spiral model does not requires the document.

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.

In waterfallmodel we will not come to know about the output 
unless all the stages are over.
In spiral model,we will know the final output almost in the 
begining itself..as it is a output driven method

In waterfall method clint and software developer well known 
about the problem.
  Whereas in the spiral model software developer does not 
aware about the actual problem.