first developing is finished.then testing will be started 
is known as waterfall model.

Water Fall Model: this is the first SDLC model developed so 
there are many disadvantages.
in waterfall maodel after completion of 1 phase, the next 
pahse starts b'cos the output of 1 phase is the input to 
the next phase.
Ex: until the release of initial build testing team is 
almost idle..they jus study the requirement docs & 
understand the req's b4 they get the build from dev team.

Srs ---> Design---> coding--->testing--->maintenance phase.

it is time consuming cos if ne changes in the functionality 
hapens or if u find ne bug then total sdlc has to b 
excecuted once again like if u wanna add ne new requirement 
it is cost effective.

manpower is wasted in this model cos the testers has to b 
idle unless they get the build from dev team

... one more point I would like to add that as name 
suggested water fall, it is always one way or one 
directional, Means after moving to another phase after 
finalizing of earlier phase, changes can not be introduced. 
The same thing happens till project ends but still it is 
widely accepted in the testing industry. Finalization of 
one phase is more important.

Dear Akram
First buy a software engineering book to avoid posting such
a basic question. I am not joking, it is mandatory for all
testers to have a software engineering book because now or
later, you are going to compete with computer science guys
who are thorough in all these things. Also you may get wrong
answers here which you may not be potent enough to judge. I
will recommend SE by Pressman.

Good Luck