Hypertext is text that has been marked up with tags to indicate its structure. Hypertext also, by definition, is capable of linking between separate documents using "hyperlinks." Plain text is not capable of this.

Hypertext browsing is the act of browsing hypertext or the interpretation of hypertext as a document -- like a website.

Hypertext transfers are transfers only consisting of hypertext.

A hypertext protocol is a protocol relating to hypertext.

The most well-known hypertext-related protocol would be the Hypertext Transfer Protocol, which is used to transfer hypertext from one machine to another.

This is a broad question and can be narrowed down by what hypertext is and why it is useful.

See the related questions of this question for "What is hypertext?" and "Why is hypertext useful?"