The original Zork was written by MIT graduate students around 1977-1979, for the DEC mainframe in use there at the time. Its core design was inspired by the game Adventure, but it quickly grew to become far larger and more complex, both in technology and content. The game was successful enough with the other MIT students that the core programmers founded Infocom and ported it to a virtual machine technology they had developed called Z-Machine. Eventually Infocom was bought by Activision, which released a few 3-D graphical adventures using the locations and backstory that had been developed. The most recent of these to be made was Zork: Grand Inquisitor. In 2009 Activision licensed the name out to Jolt Country who made a poorly received free-to-play game named Legends of Zork. It had some references to the series in terms of location names but none of the same gameplay and not much story connection. It was really part of the franchise in name only.
In the original text adventure series you played as a nameless adventurer, wandering around a deserted land once known as Frobozz once populated by people called Frobozzicans who were ruled by flat-headed leaders (literally named Flatheads); all of whom have since vanished. The only other characters you meet is a thief, who can steal from you and may kill you but does not say anything and a troll. The later 3-D adventure games (especially Grand Inquisitor) take place in other time periods and have more characters.
At the start of Zork I you are above ground, near a small white house. You can wander around in the forest a bit, and later on you float down a river and arrive at a beach, but the majority of the game takes place inside the Great Underground Empire. As the name implies, the G.U.E. was located underground in a large system of caves, accessed via a trap door inside the house. Other games in the series similarly take place in a mixture of above- and below-ground environments.
Gameplay follows the typical adventure game formula and consists of visiting the various areas and collecting the objects you find along the way. Sometimes there are puzzles to solve with the objects you have collected or portions of the environment. If you meet the thief or the troll you can engage in brief swordfights with them, which are turn-based and decided by dice rolls.