Stub  : A piece of code that simulates the activity of      
missing components. 

Driver : A piece of code that passes test cases to another 
piece of code.

i will give a gen example....suppose u have 3 modules...A B
C...A n B r 100% comp....and C is only 50% comp....u r under
pressure to comp in a time frame...what u do is u know to
build the mod C u need at least 15 days...so u build a dummy
mod C which wil take 1/2 days...This is STUB
now once all the mod A B and C(dummy) r ready..u integrate
them to see how it works..This is a DRIVER

these two are used in integration testing. 
Stub; is used in Top down integration testing to simulate 
the called module
Driver:is used in bottom up integration testing to simulate 
the calling module

At the time of integration development team finds one of 
the sub module is yet to be released from unit testing then 
they install a virtual environment is called 'stub'nd 
continues integration testing based on main module in top 
down manner
if main module is yet to be released from unit testing then 
they continues integration testing in bottom up approach by 
installing 'driver' in place of main module

Hi Manam,
           This is sunithayagnamurthy.stub and driver both 
are included in integration testing.Integrate testing means 
to test sub modules and modules.this is called integration 
testing
integration testing is divided two types
1.top down approch
2.bottom up approch
top down approch: top down approch means to test the main 
module to submodule.some times the main module is till not 
developed temparary module is created .this tempatrary 
module name called stub.It is act as main module
bottom up approach:bottomup approach to test the sub mdule 
to main module some times thesub module is till not 
developed the temparary module is created.this temparary 
module name is calles driver.It is act as submodule....
If any mistakes is thr please excuse.....
send correct answer

Stubs and Driver comes under integration testing.
integration testing means integrating the modules and 
perform testing on that.
integration testing can be performed in 3 approaches
1.Top-Down approach
2.Bottom-Up approach
3.Hybrid approach

Top-Down approach means integrating the main module with 
the sub module.during this time if some of the sub modules 
are not finished that is if they are in 
underconstruction,to continue the integration testing we 
will put one stub in the place of constructive sub module.

In bottom-up approach the integration is performed from 
bottom to up.here sub modules are integrated with main 
module.during this time if main program is not ready that 
time we will put driver instaead of main program for 
continuing integration testing once they are fully 
constructed the main program we aill replace that driver 
with the main program.

hi jyothinapa thanq for giving answer 
please explin abt hybrid approach also