Mobile first approach − Bootstrap 3, framework consists of Mobile first styles throughout the entire library instead them of in separate files.
Browser Support − It is supported by all popular browsers.
Easy to get started − With just the knowledge of HTML and CSS anyone can get started with Bootstrap. Also the Bootstrap official site has a good documentation.
Responsive design − Bootstrap's responsive CSS adjusts to Desktops, Tablets and Mobiles. More about the responsive design is in the chapter Bootstrap Responsive Design.
Provides a clean and uniform solution for building an interface for developers.
It contains beautiful and functional built-in components which are easy to customize.
It also provides web based customization.
And best of all it is an open source.
Just to give you a little excitement about Bootstrap, I'm going to give you a small conventional Bootstrap Hello World program, You can try it using Demo link.