A style guide or style manual is a set of standards for the writing and design of documents, either for general use or for a specific publication, organisation or field. The implementation of a style guide provides uniformity in style and formatting of a document.

Some style guides focus on graphic design, focusing on such topics as typography and white space. Web site style guides cover a publication’s visual and technical aspects, along with text.  Many style guides are revised periodically and/or annually to accommodate changes in conventions and usage.

Many publications (notably newspapers) use graphic design style guides to demonstrate the preferred layout and formatting of a published page. They often are extremely detailed in specifying, for example, which fonts and colors to use. Such guides allow a large design team to produce visually consistent work for the organisation.

Source – Wikipedia