Hypertext is text which is not costrained to be linear.
Hyper text is texts which contsins links to other texts. The term was coined by Ted Nelson around 1965 (see History.)
HyperMedia is a term used for hypertext which is not constrained to be text: it can include grapgics, video and sound, for example. Apparently Ted nelson was the first to use this term too.
Hyper text and HyperMedia are concepts, not products.
See also: