http://weblogs.asp.net/adilakhter/archive/2008/05/04/more-on-unit-testing-testcontext.aspx

What is TestContext?

TestContext is a abstract class of Microsoft.VisualStudio.TestTools.UnitTesting namespace that provides various information about the current test run. Purposes that has been served by TestContext Class -

1.  To store information and provide information to the unit tests during Unit Test Run.

2.  Provide a mechanism to measure performance of your code being tested by the Unit Test.

3.  In Testing the web service it stores the additional information, like server's URL.

4.  In Asp.Net unit tests, it get holds of the Page object.

5.  For Data Driven Unit Tests , it provides access to the data rows.