Watching a movie at home isn’t quite the same experience as seeing it at a movie theater -- but why? Learn how changes in aspect ratio affect every film, and why your television might not be delivering the whole picture.
The Letterbox and Widescreen Advocacy Page is a great source for easy-to-understand information about aspect ratios, widescreen, the "black bars" on the top and bottom of your screen, pan-and-scan, and so much more! http://www.widescreen.org/
For an indepth look at aspect ratios and other technical terms related to the size of your screen, visit this website: http://bitbucket.co.uk/work/aspect_ratios.html
For more information, visit this Wikipedia page. You'll find pictures, diagrams, and relevant links to help you understand aspect ratios: http://en.wikipedia.org/wiki/Aspect_ratio_%28image%29