Ok, silly question, but what is it? Is it really the id for my FB application? What if I don't have one? What if all I want to do is allow a user to log in and publish 'in-app' generated content to their wall?
Go to facebook.com/developers/ and click the "Create App" in the top-right. It's simply a portal for your App to talk with Facebook. When you create it and everything, you'll get an App ID and everything there.
I was actually just working on getting a FB App and everything. I felt the same way you did; All I wanted was a little dialog that people could post on their FB Walls for their friends to click on. It's a bit more complicated then that, but it's manageable. Much easier than Twitter, anyways :P
Think you can help? Login to answer this question!