A style guide (or manual of style) is a set of standards for the writing and design of documents, either for general use or for a specific publication, organization, or field. (It is often called a style sheet, though that term has other meanings.)
A style guide establishes and enforces style to improve communication. To do that, it ensures consistency within a document and across multiple documents and enforces best practice in usage and in language composition, visual composition, orthography and typography. For academic and technical documents, a guide may also enforce the best practice in ethics (such as authorship, research ethics, and disclosure), pedagogy (such as exposition and clarity), and compliance (technical and regulatory).
Style guides are common for general and specialized use, for the general reading and writing audience, and for students and scholars of various academic disciplines, medicine, journalism, the law, government, business, and specific industries. House style refers to the internal style manual of a particular publisher or organization.