At their most basic level they are supposed to be inspirational stories about how the author has overcome the odds to live a normal life. As K1nS says, a lot of them seem to revolve around some kind of abuse, but they can also be about illness and loss as well.
As the piece I linked to above says, the market for this type of book has grown hugely in the last few years, and most of this seems to be down to the large supermarket chains selling these books at knock-down prices.
Personally, I find the genre a bit distasteful. If you are reading the books because you have had similar experiences and they are helping you to deal with it I can see they would be useful, but I don't understand why someone with no personal insight into the situation would find them interesting, it all seems a bit, err, voyeuristic? to me.
Please don't take that personally K1nS, I