In integration testing the individual tested units are grouped as one and the interface between them is tested. It identifies the problems that occur when the individual units are combined i.e it detects the problem in interface of the two units. Integration testing is done after unit testing.
There are mainly three approaches
Top down approach tests the integration from top to bottom, it follows the architectural structure.
Example: Integration can start with GUI and the missing components will be substituted by stubs and integration will go on.
In bottom up approach testing takes place from the bottom of the control flow, the higher level components are substituted with drivers
Big bang approach
In big bang approach most or all of the developed modules are coupled together to form a complete system and then used for integration testing.