HTML 5: Offline Application Caching
All browsers have some kind of caching mechanism in place, but to be honest, they don't always work. Until HTML 4, the only work around was that the user had to save each page individually. HTML 5, thankfully, provides a smarter solution.

For the full story, see:
http://www.webreference.com/authoring/languages/html/HTML5-Application-Caching/