They do different things. Modernizr detects the availability of features in a page allowing you to provide your own polyfills for older browsers should you require that functionality. You can add support for <canvas> using a canvas tag polyfill so that canvas functionality, including it’s JavaScript interface, in browsers that don’t support the <canvas> tag. … Read more
Modernizr is a test suite that adds feature detection via classes applied to the body tag. It includes HTML5Shi(v|m) to add older browser compatibility. You don’t need Modernizr just to style new tags in older browsers. That said, it is OK to include Modernizr with Twitter Bootstrap. You can even get an auto-generated H5BP template … Read more
It was originally called the html5-shiv. Shiv really isn’t the right term, as a shiv is a stabbing-implement. A shim is something which you use to level things out (or prop them up). If a table has one leg that’s too short, you might shim it with a piece of wood or a phone book… … Read more