Zork is a text based computer game that was developed in the early 1980's. It follows the basic role-playing game adventure game system. In this game as in other games of its type, you explore the world via entering and leaving rooms. You can also pick up and manipulate certain objects with text commands. In order to play the game, the user needs to find treasure that is distributed throughout the labyrinth-like world.
The game is similar in respects to a hypertext system in the fact that the user decides where he or she will ultimately end up. Each command, like move "East", "North", "West", "South", or "What is" for instance, will display more information, much like a link does in a hypertext system. While a user in hypertext is presented with a document in its entirety, and it usually has links as well, games such as Zork will give you snippets of information, or descriptions of actions with the links acting more like paths in which the user can choose.
It is described as an "interactive fiction" which if broken down, is exactly what it is. Hypertext fictions themselves are also interactive fictions, and since the Zork allows you to choose paths much like hypertext does, it could theoretically be viewed as a sort of hypertext system. One of the key differences of course being that it is not static, as in the game may be interactively act on the user's character, as well as the user acting on it.
Since the age of Zork, many other role playing games, such as the Legend of Zelda, have been developed.
For those who would like to try Zork on Mac OS X the instructions here will help, otherwise the main site is all you will need.