What Hypertext is
A system for writing and displaying text that can be linked in multiple ways to related documents and available at several levels of detail. The term was coined by Ted Nelson to refer to a nonlinear system of information browsing and retrieval based on associative links between documents. The World Wide Web uses hypertext transfer protocol (HTTP) to link pages and multimedia files.
The meaning of Tag
In Web programming languages, it is the code that describes a command or instruction so that a Web browser will be able to interpret and display it. In order to link an image or word on a Web page, you must put specific tags around the image or word in the code. This is known as basic HTML and it's pretty easy. Look up the HTML definition for more info.