Difference is nothing Top down we start testing from amin 
module without sub module (Stubs 4 sub modules)

In Bottom up we dont have main module we start testing with 
sub module(Drivers are 4 main miodule)

Both have thier own advantages as wee as disadvantges

Can anyone explian with examples??

Generally products are developed with Bottom up approach.

Say we have have to deliver one application contains 10 
modules. One by one module is develpoed and need to be 
tested. but how will you invoke that singe module, you will 
be needing Driver to invoke it. As sooner other modules 
will finished they will be integrated with it and 
similarily you will have complete application.

In top down you will have main module is ready to 
accomodate other modules that are getting develpoed 
simultaneously.

Correct me if i m wrong