Interactive fiction tells you the beginning of a story. Then it puts you in charge and lets you decide what your character should do. You type commands for the main character to carry out, and the story replies by telling you what happens next. Think of the rooms as your setting, and the objects you find as the props.
Part of your role is to help the main character overcome obstacles to his progress: solving problems, working out what is going on in the story, discovering ways to reach new locations and tools. That aspect of interactive fiction is like a game.
And part of your role is to help him make decisions. Interactive fiction may have multiple endings, subject to your choices. That aspect is like a story — but an open-ended one.
If you enjoy interactive fiction, you’ll find a wide variety of free works available to you. These come in many styles, some more like games, some more like stories; some are challenging, some very easy; some are serious, emotionally demanding pieces, while others explore the lighter side of life. And, like books, works of interactive fiction come in assortment of genres: fantasy, mystery, horror, and science-fiction, romance and historical, surreal and slice-of-life.
To try a few well-known, novice-friendly interactive fiction works now, check out this site.
For more complete instructions, see my PDF Manual on playing IF.
And if you’re a French speaker or interested in trying French IF, you may be interested in the French translation of the same manual, which includes a list of all the standard verbs in French games.
Emily Short's Interactive Storytelling 