The Bootstrap CSS framework is a front end toolkit built in style guide form. Or basically a collection of CSS and JS code recipes call components to help developers build responsive websites quickly.
When Bootstrap was first created at Twitter it was built as a toolkit of reusable components with additional documentation and code snippets on how to use them. This helped a team of multiple developers work on a project and have a cohesive methodology on how to build layouts.
The documentation and ease of implementation, made it easy to share and reference with others, regardless of their skill level. So, the initial intent of Bootstrap was to be a living style guide documentation for a team of developers to code in the same way following a set of pre-defined rules and components.
Today, Bootstrap can be used in two main ways:
- Linking to a precompiled version via CDN or locally
- A customized build using the Sass source files On the next page (Fig. 5) I explore the pros and cons of each method and also break them into smaller sub-methods to help you decide which is best for your project.
For more information: What is Bootstrap? – StackOverflow