SGML is a language for defining document structures. It uses tags enclosed in angled brackets <likethis> to give meaning to the text which they surround, just like HTML. In fact HTML is defined using SGML.
DocBook is also an implementation of SGML. A DocBook document type declaration defines all the tags which are allowed in a DocBook document, just as the HTML DTD defines the tags allowed in a web page.
The DocBook documentation (package: docbook-doc) says that
The DocBook DTD defines structural and content-based SGML markup for computer documentation, with a primary emphasis on software documentation and related classes of technical documents.