The browser reads documents, and can fetch documents from other sources. Information providers set up hypermedia servers which browsers can get documents from. On top of these, if the server has search capabilities, the browsers will permit searches of documents and databases. The documents that the browsers display are hypertext documents. Hypertext is text with pointers to other text.
Hypermedia is a superset of hypertextit is any medium with pointers to other media.
This means that browsers might not display a text file, but might display images or sound or animations. In short, the WWW designers wisely chose not to invent yet another language technology and instead chose an existing standardized language. Initially, HTML was designed simply as a convenient way to mark up text. Shortly after its creation however, the folks at CERN got wind of SGML, and the two have been struggling to stay together. HTML and SGML serve different needs and communities.