A CSS framework is a pre-prepared software framework that is meant to allow for easier, more standards-compliant web design using the Cascading Style Sheets language. Most of these frameworks contain at least a grid. More functional frameworks also come with more features and additional JavaScript based functions, but are mostly design oriented and unobtrusive. This differentiates these from functional and full JavaScript frameworks.
Two notable and widely used examples are Bootstrap and Foundation.
CSS frameworks offer different modules and tools:
Bigger frameworks use a CSS interpreter like LESS or SASS.