Long, long ago (the precise date is unclear, but as far as much of my audience is concerned "back in the days when Jesus battled the dinosaurs" is a fair estimate), a man named William Crowther wrote a little program called Adventure. Adventure wasn't much by today's high standards of gaming. It ran on computers that had little to no graphical capabilities and was rendered entirely in text descriptions, and the only way to accomplish anything was to type simple, imperative commands like "GO WEST" or "TAKE AXE" at a prompt and hope that it understood what you were trying to do. The object of the game was to explore a textual simulation of a massive Kentucky cave system—a slightly embellished simulation, unless it was in fact possible to find angry dwarves and giant snakes in Kentucky in the '70's—and collect as much treasure as you could get your hands on without falling into a pit and smashing your head open. Navigation was confusing, puzzles could be tricky, there were two sprawling mazes to get lost in and the aforementioned pits were distressingly commonplace. But people played it, and they loved it, and the genre of games known as "text adventures" was born.
For a while, text adventures were predominantly commercial affairs, made by companies like Infocom and Level 9 that produced stories and adventures in all sorts of genres–the best known, even to those unfamiliar with this genre, are probably Infocom's Zork games, the ones responsible for introducing the world to the man-eating, light-fearing grues and a few other things that have slipped into common fantasy parody parlance. They dominated the gaming market, people spent hours staring at command prompts and trying to guess what to type to open a locker or catch a Babel fish, and a good time was had by all. But then graphical adventures (like LucasArts's or Sierra's point and click games) found their way into the limelight, followed by games that focused less on complex puzzle-solving and more on other aspects of gaming. Computers got better and better and became relatively inexpensive, and people were no longer satisfied with simple text when the aforementioned graphical games looked more and more inviting and sparkly by the minute. The market for text adventures dwindled, most of the companies that produced it were bought out or went out of business, and the text adventure genre faded into obscurity.
But text adventures didn't just die. Many of the players who loved them so continued to play them, continued to enjoy them because they still exercised a few gaming muscles that the graphics-dependent games of the day never seemed to bother with. There was still something exciting about trying to stumble one's way out of the dark before being eaten by a grue. Gaming companies weren't making text adventures any longer, but its original fans still wanted more–and if they couldn't purchase them, they'd just have to make them themselves.
Nowadays there are online repositories and archives full of amateur-made text adventures, or "interactive fiction" as it's generally called today, and a wide variety of IF authoring systems that make it possible to create IF of your own without having to reinvent the wheel, all available for free download (though there are some commercial works available, and at least one well-known member of the IF community recently quit his dayjob to make commercial IF full-time). Playing IF also usually requires downloading an interpreter, the virtual machine software necessary to play games made in a specific format, though you only need one interpreter to play any and all games in the corresponding format. The interpreter loads the game and displays that infamous, intimidating ">" command prompt, just waiting for you to type your first command and take your first foray into the author's world. It all seems deceptively simple, but a quick look behind the scenes at any decent authoring system will reveal a pretty powerful engine, and taking a few games for a test run can show you some complex, involved games that, when made well, require quite a bit of creative thought to solve–or, in the case of some "non-game" titles, perform feats you wouldn't expect from programs the likes of Adventure. I've only just started dabbling in these sorts of things, and let me tell you, man, mind blown.