Zork

Zork was one of the first interactive fiction computer games and a descendant of Colossal Cave Adventure. The first version of Zork was written in 1977-1979.
"Zork" was originally MIT hacker slang for an unfinished program. The creators named the completed game Dungeon, but changed it back to Zork after receiving a trademark violation notice from the publisher of Dungeons & Dragons. Zork has also been adapted to a book series.
Three of the original Zork programmers joined with others to found Infocom in 1979. That company adapted Zork into Zork I-III, a trilogy of games for most popular small computers of the era. Personal Software published what would become the first part of the trilogy under the name Zork when it was first released in 1980, but Infocom later handled the distribution of that game and their later games. Part of the reason for splitting Zork into three different games was that, at the time, micros did not have enough memory and disk storage to handle the entirety of the original game. In the process, more content was added to Zork to make each game stand on its own. A version of Zork I was issued as a hidden 'easter egg' in Call of Duty: Black Ops. It has exactly the same text and rules as the original.
Setting
Zork is set in "the ruins of an ancient empire lying far underground". The player is a nameless adventurer "who is venturing into this dangerous land in search of wealth and adventure". The goal is to return from the "Great Underground Empire" alive with the treasures, ultimately inheriting the title of Dungeon Master. The dungeons are stocked with many novel creatures, objects and locations, among them grues, zorkmids, and Flood Control Dam #3--all of which are referenced by subsequent Infocom text adventures.
FrobozzCo International is a fictional monopoly from the game. FrobozzCo products are littered throughout all Zork games, often with humorous effect.
Zork series
The original Zork Trilogy
Zork I: The Great Underground Empire (1980, Infocom)
Zork II: The Wizard of Frobozz (1981, Infocom)
Zork III: The Dungeon Master (1982, Infocom)
Later additions to the series
All these are text-only unless otherwise noted.
The Enchanter trilogy:
Enchanter (1983, Infocom)
Sorcerer (1984, Infocom)
Spellbreaker (1985, Infocom)
Games that take place somewhere in the Zork universe:
Wishbringer: The Magick Stone of Dreams (1985, Infocom)
The Zork Quest series:
Zork Quest: Assault on Egreth Castle (1988, Infocom, interactive computer comic book)
Zork Quest: The Crystal of Doom (1989, Infocom, interactive computer comic book)
The Zork Anthology comprises the original Zork Trilogy plus:
Beyond Zork: The Coconut of Quendor (1987, Infocom)
Zork Zero: The Revenge of Megaboz (1988, Infocom, text with some graphics)
After a six year hiatus, the following games were produced:
Return to Zork (1993, Infocom/Activision, graphical)
The Philosopher's Stone (Activision, unfinished text prequel to Zork Nemesis)
Zork Nemesis: The Forbidden Lands (1996, Activision, graphical)
Zork: The Undiscovered Underground (1997, written by Michael Berlyn and Marc Blank (original Infocom implementors) and released by Activision to promote the release of Zork Grand Inquisitor)
Zork Grand Inquisitor (1997, Activision, graphical)
Related pages
Legends of Zork, an online, browser-based spin-off
69105, a number that became somewhat of an in-joke in several Infocom games
The Lurking Horror, another Infocom IF, that references Zork.
Grue, the infamous Zork monster
The Meteor, the Stone and a Long Glass of Sherbet, the winner of the 1996 Interactive Fiction Competition, is strongly influenced by the Zork universe and includes many elements.