Hypertext - Hypertext is basically the same as regular text - it can be stored, read, searched, or edited - with an important exception: hypertext is text with pointers to other text. The browsers let you deal with the pointers in a transparent way -- select the pointer, and you are presented with the text that is pointed to.
Hypermedia - Hypermedia is a superset of hypertext. Hypermedia documents contain links not only to other pieces of text, but also to other forms of media - sounds, images, and movies. Images themselves can be selected to link to sounds or documents. This means that browsers might not display a text file, but might display images or sound or animations. Hypermedia simply combines hypertext and multimedia.
