The official specification of the DITA language is available from OASIS.

OASIS DITA Version 1.1. Language Specification

The language reference describes the elements that comprise the topic DTD and its initial, information-typed descendents: concept, reference, task, and glossentry. It also describes the DITA map DTD and its current specialization (bookmap), as well as various topic and map based DITA domains.

The language specification is available online as a set of HTML files, and can be downloaded as a set of DITA XML files. The DITA files have been compiled into an Eclipse Infocenter help system that is online at DITA Infocenter.  The Infocenter is indexed, has a table of contents, and can be full-text searched.

The language specification has been converted to MediaWiki format on the DITA Wiki. There you can add comments to the language element definitions.