What Janet said. Basically a journal is a thread for you to record your individual progress any way you like, a milestone is a major accomplishment along the way, and the success story is for when you've pretty much achieved your goal.

I was already about 40% of the way to goal when I first registered here and started my journal. I did it because I wanted to discuss the subject more than my friends and family were prepared to listen by that point, so it was as much about giving them a break as anything else! I really just wanted an outlet to help keep me on track; I still had such a long way to go, and as a lifelong fatty this was all very much unknown territory for me. What better way to discover this new me than one day at a time?

Whatever you want to write in your own journal is up to you, subject to the rules Janet linked to, of course. I have no regrets about having started mine.