Let's start with a few sources of the concepts of Responsive Web Design:
Responsive Web Design
http://en.wikipedia.org/wiki/Responsive_Web_Design
Ethan Marcotte wrote an excellent book that kicked off concept and buzzword of Responsive Web Design. It's a very good read:
http://www.abookapart.com/products/responsive-web-design
Mobile First
Luke Wroblewski wrote a related book in optimizing websites for mobile technologies first to ensure that mobile users get the best experience. I highly recommend this book as well: