Twitter Bootstrap is a free set of tools for building web applications. It comprises template files, as well as optional JavaScript extensions. You need it if you're a web template designer
Twitter Bootstrap is a free set of tools for building web apps. It comprises template files and optional JavaScript extensions. Actually, it was originally designed to help provide some consistency across tools.
Bootstrap is a CSS framework that comes with an assortment of pre-built libraries, templates, and grids. It allows for users to code more efficiently and faster without having to put a ton of time trying to rebuild the wheel.
Why you didn't do a simple google search to find the answer? It looks weird to find these kind of questions at a forum like this.