When is it appropriate to use Parenthesis in your work?

Ok, the question is already established so lets find out. Here is what I think, say you are going along and writing your book, paper, essay, or what not and you want to add a little side note, "I stepped back from the door once I burned my hand on the door knob (I assumed that there was a fire on the other side)." k, Maybe not the best example. I imagine you would use the parenthisis marks to add a thought to what has already been said. They will also be used if you are referencing someone elses work, (Shamo, 8).
To the Net. I think THIS PAGE says most of what I wanted to get out of it. Short and simple.