A CSS framework is a collection of CSS files containing predefined rules for layout, typography and other CSS resets. if you are a developer you might hear of terms like boilerplates, grid systems etc. Each one of them promising more than the next one. For the most part, they all refers to a collection of assets that are design to speed up the development. CSS frameworks focused on responsive web design aspects. CSS framework cover all area of a responsive web development like typography, user interface layout etc. A good CSS framework may contain a grid system, Typography styles, CSS resets, layout models, responsive media and browser fixes. You don’t need to write a code from scratch, frameworks will provide codes for basic elements.

Take a look here: