DOCBOOK
DocBook is an OASIS standard that provides a system for writing structured documents in SGML or XML. It is a large, robust, and easy-to-understand DTD, the main structures of which correspond to a book. There is also a small subset DTD, called simplified DocBook, for articles.
It is especially well-suited to books and papers about computer hardware and software, but not limited to them. Simplified DocBook is well-suited for shorter, article-length material.
DITA
Initially developed by IBM, the DITA DTD is now an OASIS standard.
DITA defines
A set of document types for authoring and organizing topic-oriented information in XML.
A set of mechanisms for combining and extending document types using a process called specialization.
DITA is best suited for writing modular, topic-oriented documents.