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.