What is the purpose of unit testing? Who performs it? How is the test data prepared? What is the difference between unit testing and integration testing?? What are the objectives

1. What is the purpose of unit testing? Who performs it? How is the test data prepared?
2. What is the difference between unit testing and integration testing? 
3. What are the objectives of integration testing?
4. What distinguishes integration testing from system testing?   What kinds of tests can be included in
   system testing?

Pick two of the above questions and discuss each as a posting