DisAdvatages

1.  Inflexible partitioning of the project into distinct 
stages makes it difficult to respond to changing customer 
requirements.
        2. The lifecycle can take so long that the original 
requirements may no longer be valid by the time the system 
is implemented. 
3.	Estimating time and costs is difficult for each 
stage. 

actually the waterfall model is a dependent model.
so in the every phase depends upon another phase.So after 
completion one phase(sdlc) then goes to next phase.
for example after completion of coding testing should 
happen.it should dependent.
If u want to add any new requirement it should be so cost 
effective.

advantage is cheaper.

Advantages:
Model is simple.
Easy to understand.

Disadvantages:
Time consuming as the testing process starts the last  
stages
till the build is given for testing the testers will be 
idle.
hence man power is wasted

Advantages:
      it is simple and easy to understand.
      it takes less cost.
disadvantages:
      in this stage every phase requirements are freezed.
      we can't do modifications.

disadvantages:

time waste,
hard to execute

it lacks overlapping and interaction among the phases

Advantages of Waterfall Model 
1. Clear project objectives. 
2. Stable project requirements. 
3. Progress of system is measurable. 
4. Strict sign-off requirements. 

Disadvantages of Waterfall Model 
1. Time consuming 
2. Never backward (Traditional) 
3. Little room for iteration 
4. Difficulty responding to changes