Testing in which software components, hardware components, or both together are combined and tested to evaluate interactions between them.
Integration testing takes as its input modules that have been checked out by unit testing, 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.
                           
What:
Testing performed to expose faults in the interfaces and in the interaction between integrated components.

Who will do:
As integration testing covers broad length of ‘V’ model. From the developers to System integrators will involve.

Where it:
Almost all integration will takes place at the developing organization sites.


Integration testing can be conducted in two ways.
1) Non-Incremental Approach
2) Incremental Approach

1) Non-Incremental Approach:
The non incremental approach is also known as “Big-Bang” testing. This approach is very unfashionable due to the level of risk that one takes in hoping that the system will perform as expected.

2) Incremental Approach:
Incrementally integration testing can be split in to
a) Top-down Testing
b) Bottom-up Testing
c) Sandwich (Hybrid) Testing
                             
a) Top-down Testing:
In Top-down Testing, higher level modules are tested. If lower modules required to make up the system are not yet available then, stubs are used to simulate their activity.

Stub: Small software placed in to a program that provides a common function.

b) Bottom-up Testing:
In Bottom-up testing, lower level modules are tested. If the higher level modules required to make up the system are not yet available then, drivers are used to simulate their activity.

Driver: Drivers are simple program designed specifically for testing that make calls to these lower layers.