According to my dictionary, a prologue is a separate introductory section of a literary work. An event or action that leads to another event or situation (civil unrest in a few isolated villages became the prologue to widespread rebellion). A short preliminary time trial held before a race to establish a leader (in professional cycling). The actor who delivers the prologue of a play.
Two of those definitions can be used in your book. Now a lot of those comments indicated that information in a prologue should simply be included in the story in some way, but sometime that just isn't possible. Sometimes the information is vital to setting the stage in some manner, but it just doesn't fit anywhere within the story, not unless you want to commit the ultimate sin and include a lot of back-story somewhere.
In my first book, and in my latest book, Half-Breed, setting the stage was important to me. I needed to push the setting of both books into the far future of this earth we walk today. I didn't want these two stories to be just any old where. The lack of a moon is a culture changer, but if no one knew that the world had once had a moon, they would miss picking up on that small item; it wouldn't be a culture change, it would only be different. There were other small things in both books that I wanted to be seen as the same but different, not just other. Lack of prologues would have allowed them to be alternate Earth stories, but I wanted them to be tied much closer to home.
So really, I suppose it just boils down to 'follow your gut'. Use prologues with extreme care; consider all the alternatives first, and then consider exactly how important that information really is. Consider VERY carefully before plugging a prologue in front of your story, because, like I said before, some people will skip over it anyway. Ask yourself, "what does this story mean without the prologue?"
And then lets not forget the prologue's littlest cousin, the epilogue.
According to my dictionary, an epilogue is a section at the end of a book that serves as a comment or conclusion to what has happened.
I used one of those too in my first book. I had so many threads going through that book that when I reached the desired end, many of those threads were left hanging. I felt the need to wrap things up. I did a much better job of wrapping things up in Half-Breed but I still included an epilogue. I mean, the guy simply had to see his kids even though the story had reached it's conclusion.
In The Fortunes of Magic, the last chapter could have been an epilogue after a fashion, because it was a shift of POV, but really that chapter didn't fit the definition above. It was my way of bringing things full circle in that story. Same with The Speed of Dreams. The last chapter was a bring-around to a full circle. Neither book had a prologue either.
How about you? Do you favor a prologue? Have you ever used one? How about the epilogue?