PostgreSQL has four primary documentation formats:
Plain text, for pre-installation information
HTML, for on-line browsing and reference
Postscript, for printing
man pages, for quick reference.
Additionally, a number of plain-text README-type files can be found throughout the PostgreSQL source tree, documenting various implementation issues.
The documentation is organized into several "books":
Tutorial: introduction for new users
User's Guide: documents the SQL implementation
Reference Manual: reference pages for programs and SQL commands
Administrator's Guide: installation and server maintenance
Programmer's Guide: programming client applications and server extensions
Developer's Guide: assorted information for developers of PostgreSQL proper
All books are available as HTML and Postscript. The Reference Manual contains reference entries which are also shipped as man pages.
HTML documentation and man pages are part of a standard distribution and are installed by default. Postscript format documentation is available separately for download.
The documentation sources are written in DocBook, which is a markup language superficially similar to HTML. Both of these languages are applications of the Standard Generalized Markup Language, SGML, which is essentially a language for describing other languages. In what follows, the terms DocBook and SGML are both used, but technically they are not interchangeable.
DocBook allows an author to specify the structure and content of a technical document without worrying about presentation details. A document style defines how that content is rendered into one of several final forms. DocBook is maintained by the OASIS group. The official DocBook site has good introductory and reference documentation and a complete O'Reilly book for your online reading pleasure. The FreeBSD Documentation Project also uses DocBook and has some good information, including a number of style guidelines that might be worth considering.