What is DocBook?
DocBook is an open standard for creating books, papers, articles, and other informational resources that contain text and media. It is particularly well suited to materials about computer technologies, though it is not limited to these applications.
In a nutshell, DocBook defines the data interface (schema) for documents. All documents that follow the schema can then be rendered to a publishable medium be it a printable book, a PDF file, Markdown or an HTML website.
DocBook is a mature technology that has been in development since 1991. Initially initiated as a collaborative effort between HAL Computer Systems and O'Reilly & Associates, DocBook eventually established its own maintenance organization. Today, it is overseen by the DocBook Technical Committee at OASIS.
DocBook predates XML, which accounts for its continued reliance on Standard Generalized Markup Language (SGML). As a result, you can still find SGML definitions in the contemporary DocBook versions. This blessful quirk played a positive role in DocBook development making it a polyglot environment. DocBook supports several languages (SGML, XML) that can be used by authors for material creation, making the technology a truly future-proof.
Another distinctive feature of DocBook is expressiveness and richness. While there are plenty of other emerging standards nowadays, DocBook is probably the most expressive of them. DocBook provides a productive environment for authors by giving them plenty of proven and production-ready primitives they can rely on. If you never wrote a book before, DocBook gives you a solid basis to get started.