Unit Testing

Types of tests?

Load or Stress test (performance)
Function or System tests (manual)
User Acceptance testing (by customer)
Integration test (testing interacting components of the system)

Unit test is basic level test designed to test your s/w at the smallest level.In the concept of OOP's unit testing is done at the method levels and tests the method at the single level class.
As the complexity of the s/w grows there are more chances to erroer in it.

Unit testing has come up within VS2008(Professional edition) with MSTest.If you are using any other edition then you won't have it within.MStest has test menu in VS2008 also we have test project in VS2008.
Other popular 3rd party softwares are NUnit.most popular and it can be downloaded from http://www.nunit.org/index.php
other is MBUNIT http://www.mbunit.com/
XUNIT http://www.codeplex.com/xunit

Now with each software we get the runner which shows us the test results.