- A CSS reset
- A grid system for quickly creating layouts with responsive design.
- preset styles for typography, spacing, and widgets like buttons and navigation menus
- Sets of icons
- CSS helper classes like “hide” or “left"
Many modern libraries also add support for CSS preprocessors like SASS, LESS and Stylus.
There are many, many competing CSS frameworks, that differ in both their look and approach. The biggest names at the moment (11/17) are Twitter Bootstrap, Semantic UI and Foundation. Personally, I find Tachyons interesting for their DRY approach.
Some CSS libraries only fulfill one of the above features, like the popular 960 grid system.
CSS libraries are massively popular because many developers hate wasting time writing CSS. Frameworks let you quickly mock up an app that will look reasonably good without running into the inevitable oddities and frustration of working with CSS. In theory, anyway.
Plus, adopting a CSS library can cause you pain down the road if you ever want to get rid of it: some compare using bootstrap to taking out a mortgage. Worse, many developers use CSS frameworks as a crutch that prevents them from needing to really understand CSS, leading to sloppy and broken code.
First, take the time to truly understand CSS. Then, do some research and pick a framework that you like. Often, the best way to learn to use it is by reading the official documentation. Alternatively, there are tutorials and classes out there for the most popular frameworks, such as: