If you're looking for some good working definitions and explanations about what SOA is all about, ServiceOrientation.org is a good place to start.
Created by Thomas Erl, author of Service-Oriented Architecture: A Field Guide to Integrating XML & Web Services,the site provides nice brief, to-the-point overviews (and some charts)of concepts such as service reusability, contracts, loose coupling, andabstraction.
The site also includes a bunch of good quotesabout SOA. A favorite is from Microsoft's Mike Burner: "Presented withan egg, a farmer might envision a chick; a cookmight envision an omelet; and a child might envision a brightly paintedEaster decoration. Service orientation is an egg."