CSS frame work are pre-prepared libraries that are meant to allow for easier, more std-compliant styling of web pages using the Cascading Style Sheets language. Layout-grid related CSS frameworks include blueprint ,960 grid etc. Like programming and scripting language libraries, CSS frameworks are usually incorporated as external .css sheets referenced in the HTML <head>. They provide a number of ready-made options for designing and laying out the web page. While many of these frameworks have been published, some authors use them mostly for rapid prototyping, or for learning from, and prefer to 'handcraft' CSS that is appropriate to each published site without the design, maintenance and download overhead of having many unused features in the site's styling.
I rather use "reset" stylesheet and create the whole file from scratch. Lately I found a new library - normalize.css. It leaves default formatting (h1 with the biggest font-size etc.), and works in all browsers the same way.