Zork is a text adventure, which is a form of interactive fiction, like a cross between a novel and an RPG with some escape-the-room type puzzles thrown in.
The very first text adventure was simply called Adventure and it was fantasy based. Most of its influences came from J.R.R. Tolkien and Dungeons and Dragons. The heyday of the text adventure was before computers could handle graphics, as we are used to today. Instead, intricately detailed dungeons with twisty little passages were all described with text, thus leaving much to the imagination of the player. If you have participated in forum-based role-playing then you have experienced a form, possibly even a derivative, of the text adventure. The difference is, in a text adventure you are the only player and the moderator is the computer.
In the case of Zork, it was one of the first text adventures and even spawned several sequels. Originally conceived in the late 1970's by a research group of MIT, the game was written by Tim Anderson, Mark Blank, Bruce Daniels, and Dave Lebling. And now, many years later, the first game of the Zork series has been ported to PHP for your gaming pleasure by someone who goes by kodrik over at thcnet.net. The game is, in itself, somewhat self-explanatory; the trick is getting the text parser to understand your commands enough to tell you what you want to know. I've prepared a list of useful things you should know before playing Zork:
Help - If you've never played a text adventure before, just type help and the game will give you a quick tutorial. I am not going to provide an in-depth tutorial about text adventures here since the good folks over at Brass Lantern have already done that for you.
N, S, E, W - If you want to move in this game, type n for north, e for east, w for west, and s for south. The game will describe the directions you are permitted to go. It will tell you, for example, that there are "Exits to the north and East."
Get (or take) - The command for taking things, such as "get key," all objects near where you are will be described to you, though you may have to poke around to 'see' everything.
Look - If you just type 'look,' you will be given an explanation of where you are and what's around you. 'Look' can also be used in conjunction with other things, such as "look at leaflet."
Save - Saves a game in progress.
Restore - Restores a previously saved game.
There are many other commands as well, such as open, close, read, lift, pull, etc. Just type, in easy-to-understand language, what you think you should be doing, and you'll get the hang of it. Other than that, good luck.
If you're really into the TA scene, be sure to check out Wulfo's review of interactive fiction by Zarf.
And, even more fun, is an unbelievable easy-to-learn text adventure creation program called Quest, which was developed by a British programmer named Ray Pang. Yes, it does require a download, but it's worth it. You can even make online multiplayer games and submit your own creations to the site. There are quite a few good ones already there; I suggest playing Mansion and The Former, in particular (the latter being submitted by yours truly). The program comes with a tutorial and this post has already rambled on for long enough, so I guess you're on your own with this one. But trust me, it's worth a look.