Salesforce Lighting Design System(SLDS) is a CSS framework introduced by Salesforce to create faster,responsive and beautiful pages. This css framework is similar bootstrap but have advantages over it if you are working on Salesforce platform.It is a pure css framework that you can use with any front-end development framework like Visualforce, Lightning pages and html pages.
Visualforce page with lightning design system automatically fits to Device size and pages can be create and used across all devices.