Interactive fiction, formerly known as text adventures, are almost purely text based games where you type the commands for your character, and get the responses as descriptions of items, your surroundings and the actions.
To play these games you’ll need an interpreter, which is like a music player for mp3-files. You can find more information in the IFWiki, but I’ve found the following to be ok.
So, go and download an interpreter and use it to open the game file. You can find tons of more games in the Interactive Fiction Database.
Mac OS X: Zoom
Windows: Windows Frotz 2002