TDD

Test Driven Development (TDD) is a lightweight programming methodology, in witch the tests are implemented before writing code. The steps are very simple:
1) Write a small test
2) Run the test
3) Write enough code to make the test succeed
4) Refactor: Clean up the code
5) Repeat
TDD Mantra
The TDD mantra is very simple: red/green/refactor (like a traffic light).
Red - Test Fails
Green - Test succeeds
Refactor - Clean up
This is the xUnit terminology.

xUnit can be seen has the language for writting tests. The xUnit is a framework that makes possible to define assertions.
xUnit is implemented in all main computer languages. The first was sUnit, for Smalltalk.
There are others like nUnit, clrUnit, and csUnit for Microsoft .Net framework, jUnit in Java and vbUnit for Visual Basic.

References:
http://www.testdriven.com/
http://www.xprogramming.com/

xUnit:
nUnit
csUnit
clrUnit
jUnit
vbUnit
sUnit
VB Lite Unit
Open Source Testing: The open source testing site
http://www.opensourcetesting.org/