Integration Testing: It is the phase of software testing in which individual software modules are combined and tested as a group. It follows unit testing and precedes system testing. In simple, terms combining individual modules and performing the test on those modules is called integration testing.
Integration testing is actually composed of different types of tests, but its objective is to ensure that the interaction of two or more components produces results taht satisfy functional testing requirements.
For Example:
Let A and B be two components in which A calls B.    Let Ta be the component level tests of A    Let Tb be the component level tests of B    Tab The tests in A's suite that cause A to call B.    Tbsa The tests in B's suite for which it is possible to sensitize A            -- the inputs are to A, not B.    Tbsa + Tab == the integration test suite (+ = union).
Actally there are 2 approach to perform Integration Testing.
Top Down and Bottom Up Approach.
Use Top Down if you are working for an project that has been already been developed and use Bottom Up Approach if you are working for the Development Project.
Testing of 2 or more than 2 modules of application/applications is integration testing. It involves validation of data flow via xml or any other technology in between modules.
