By The post isn’t titled particular well and it doesn’t go into Agile processes, but its a great description of the underlying principles and the wider environmental reasons for them. Its ‘what’ and ‘why’ rather than ‘how’.
The text of this post is worth a read but the real reason I’m throwing this link out there is the diagrams. You wont really understand Agile until you’ve used it (and succeeded), I have a couple of projects under my belt and I’m still learning (although by its very nature, with Agile you’ll always be learning). However looking at these diagrams I think they would have helped me to visualise Agile during that initial learning phase. I have a feeling they would be great as slides in an ‘introduction to Agile’ presentation.
http://sixrevisions.com/web-development/agile/