I find that the tenets of the Manifesto for Agile Software Development ring true in my software development experience:

We are uncovering better ways of developing software by doing it and helping others do it. Through this work we have come to value:

• Individuals and interactions over processes and tools
• Working software over comprehensive documentation
• Customer collaboration over contract negotiation
• Responding to change over following a plan

That is, while there is value in the items on the right, we value the items on the left more.

Scott Ambler's site agilemodeling.com has a few good introductory articles. The Agile Alliance site has links to many more articles from different sources.

I seem to recall seeing an article when I found these pages in which the author promoted the idea of writing as few test cases as possible before writing code (contrary to standard extreme programming practice). Instead, he emphasized writing a test case for each debugging session/bug fix and saving each of those. Three weeks later, I can't find this reference. Please let me know if you see it.