Integration testing (sometimes called integration and testing, abbreviated I&T) is the phase in software testing in which individual software modules are combined and tested as a group. It occurs after unit testing and before validation testing. Integration testing takes as its input modules that have been unit tested, groups them in larger aggregates, applies tests defined in an integration test plan to those aggregates, and delivers as its output the integrated system ready for system testing.
Stub is a temporary similar module like sub module used in top-down approach
Driver is a temporary module which acts like main module used in Bottom-up Approach.
Top Down Testing
An integration testing technique that test the high-level components first using STUBS for lower-level called components that have not yet been integrated and that stimulate the required actions of those components.
In top down approach, a core module is developed. To test that core module, small dummy modules are used. So stubs are small dummy modules that test the core module.
Bottom Up Testing
An integration testing technique that tests the low level components first using test DRIVERS for those components that have not yet been developed to call the low level components for test.
In bottom up approach, small modules are developed. To test them a dummy core module Is called driver.