Zork was the creation of some students at MIT (chiefly Marc Blank and David Lebling). The word "Zork" was a nonsense word in circulation at the time. Zork was written in MDL, and ran on a mainframe computer, to which people could dial in to play the game (other popular games of this ilk included Advent and various Star Trek games).

The micro-computer version of Zork was based upon the first third of the mainframe Zork map (the two sequels obviously were made from the remainder). To my knowledge, it was the first commercially available adventure game. The home version of Zork ran on the now legendary Z-Machine. The Z-Machine was a real Zefram Cochrane style breakthrough - it was the first computer program anywhere to use virtual memory, for instance.

It is interesting to note how each location in Zork shows off a new feature, pretty much in the order that they were added to the original game. (For instance the lamp shows a universal property of light and dark, the bucket is a closable container, the crack in the wall is a unidirectional - or rather a conditional - link, the trapdoor under the rug is a hidden object, the sack is a container, the thief is a moving object).

Originally when you ventured into darkness you were likely to be killed by falling rocks - the grues were added to allow non-subterranean locations. The strangest thing about Zork is the lack of any coherent plot or setting. Another oddity (and an extreme irritant) is that some of the puzzles rely on you knowing completely arbitrary "magic words" (such as typing ODYSSEUS to scare away a cyclops). But hey, it was the 70's. We can forgive them a few mis-steps.

Everything you could ever want to know about Zork and Infocom is organised here: http://www.crosswinds.net/~dsinclair/index.html

Excellently, you can play mainframe Zork through your browser here: http://thcnet.net/error/404.php

(Seriously, all 404 error pages should carry classic 1970's interactive fiction games.)