Zork is a classic in the genre known as adventure games, which are about exploring strange places, solving mysterious puzzles, and surviving bizarre scenarios. Other classic examples of this genre include King's Quest and Monkey Island.
Zork has a few things that make it unique from those later games, though. It is actually one of the oldest computer game franchises around, with the first game making its public debut in 1980. Because of this, newcomers to the series might be shocked to discover upon loading up the original game that there's not a graphic in sight. The early games in the series are an example of a sub-genre of adventures known as text adventures or interactive fiction. What this means is that the game takes place in a turn-based environment where the game will display text on the screen as if you were reading a book, and then you, playing the part of the main character of the story, will type in what you want your character to do next in the story. It's sort of like a Choose-Your-Own Adventure book, but much more open to exploration and experimentation.
Later games did eventually shift into a graphics-based system, but the hardcore fans still remember the text-based games fondly.
If you are interested in trying out Zork, I recommend you go to www.gog.com
, which is a digital distribution website for games, like Steam except they specialize in re-publishing classic games running on modern hardware. At that site, if you sign up for an account (which is absolutely free), you can purchase several of the Zork games. If you are not interested in the text-based concept, I recommend you start with Zork Grand Inquisitor, arguably the best of the Zork graphical games. If you do wish to take the plunge into hardcore territory, there is a collection of some of the text adventure games available as well.