It's like those "Choose Your Own Adventure" series. At the end of each chapter, you're given a choice, and you can choose which path you want to take. If you come upon an empty entry, you can fill in whatever you want and give the next person their two choices.
Read away and add to the newest story! Go! Go! Go!
Below is a previous interactive story that's too huge for anyone to actually spend their time reading all the branches, so I closed it down. Still interesting to read the stuff people put in it though. I haven't actually read all of the branches (well to be honest, I haven't even read a quarter of them), so um... hopefully there isn't anything inappropriate that I didn't catch and remove. Heh.