It is interesting in that the succession follows individual lines rather than a notion of seniority. That is, after Elizabeth, Charles is next. After Charles, his eldest son William. This could all be guessed. However, if something were to happen with William, one might think it would go to the next eldest of Charles' sons - Harry - but that is not how it works. The crown trickles down through the children of William, then their children and so forth. I am not an expert, but it used to be the sons were considered first, by age, and only after all the sons were exhausted would the crown go to the eldest daughter and so forth. But I think that has changed recently, with sons and daughters being considered on equal footing when it comes to succession. To see this in action, consider that all of Charles' brothers "trump" Princess Anne, although she is the eldest after Charles. However, when you look at the pecking order of the children of prince William, they are right in line by age. Anyway, Harry is sixth in line now but over time that could easily go to double-digits. The chances of he becoming King are quite remote.
Religion, divorce, religion of spouse...all this plays into succession, with changes being made over time.
Corrections welcome!