What is Modernizr.js?
Modernizr is a javascript library that detects whether or not a browser supports certain HTML5, CSS3, or other elements. Then, it gives you CSS classes to control what happens if the browser doesn’t support those elements.
When to Use Modernizr
Chris Coyier, from css-tricks.com, explains when (and how) to use Moderizr in a simple, easy-to-understand way.