The MSDN (Visual Studio) definition for Unit Testing is brief and to the point. A good reference for the primary goal and answer to the interview question: What is Unit testing?

The Wikipedia entry for Unit Testing gives a good definition of Unit Testing. The Wikipedia entry also describes test driven approaches to software design as well as Extreme Programming which relies on an automated testing framework. Altogether a good web reference for Unit Testing concepts and definition.

If you are looking for an overview of TestNG, together with its advantages over Junit, that is independent from the reference sit then this IBM resource should fill the need. This article gives a brief history and explanation of the TestNG framework then dives into a quick start example that exemplifies the advantages of TestNG over Junit.

This article gives an overview of Junit as well as a step by step guide on how to use Junit within the popular open source Eclipse IDE. There are plenty of screen shots and detailed steps in order to take anyone through their first steps of creating a Junit project in Eclipse.

This is a Chapter about testing with Junit that is contained in an online Java programming book. This chapter offers a primer on Junit and is a useful resource to get an overview of the basic concepts and get started using the framework itself. There is also a section on generating test reports (from XML output) using junitreport. The DBUnit framework is also referenced in context of establishing any data dependencies for a given Junit test. 

As we look through the myriad of testing frameworks (and combinations) it is worth taking a step back and asking what are my requirements (for a testing lab) and proceed from there. The Selenium and TestNG combination is worth a look for anyone considering writing data driven tests for web apps.