An Interactive Adventure is a story/game in which the narrator moves the plot forward based on the suggestions of other members.
The Interactive Adventure can take place in any setting. Similarly, it could be any genre. It is entirely up to the narrator how the adventure begins. After that, the narrator and the readers must work together to continue forward. The narrator may use whatever mediums he feels comfortable with, such as writing, drawn images, photos, flash, etc.
These adventures can be as open-ended as the narrator wishes it to be. He or she can use all, some, or none of the suggestions given (though if you never use a suggestion, it is not Interactive). A narrator who wishes to present set choices for the readers, may do so (such as in a Choice A, Choice B, Choice C format). The narrator may add any additional rules to his or her adventure.
To create an Interactive Adventure, just post a thread in the Pokemon Fan Fiction or Other Fan Fiction section depending on whether it is Pokemon-related or not. Be sure to add the [IA] tag in front of it to let others know it's an Adventure.
To participate in an adventure, the only thing you need to do is post a suggestion. The suggestion can range from the serious to the hilarious, though it is the narrator's decision which suggestions are accepted.
Feel free to ask any questions or clarify any points in this thread.