A style guide is a tool that outlines the styles a brand will use. These styles can be anything from logo, color and typeface usage, to editorial voice, to the very specific ways that interface elements will be designed. Style guides help a website maintain a consistent user experience as the site is updated or expanded.
Style guides provide direction and constraints for a team of designers, writers, or coders. Even a solitary designer can benefit from creating a style guide for a project, and sticking to it.
A huge list of Style Guides and UI Guidelines by Abhay Rautela