According to the Gita (The Popular Religious Text Of India) the cycle of birth and death is somewhat interesting. Basically every soul from "Godhead" starts into the cycle of birth and death when it departs from God. You start off as pretty lowly things and build up to more intelligent creatures "karma-free" until you become human. When you become human the rules change, you are no longer karma free because you are capable of making choices and being taught right from wrong, greed from generosity etc.
SO. If you don't live your human life right the first time around, IE you don't connect with God and devote your life to that connection, this includes sooo many things, the simplest of which is compassion to all animal life, since animals are souls working their way back to God. Umm. what was I saying oh yes you will repeat human life again.
If you we're REALLY bad you regress to an animal again and increase the length of the journey back to God. You do it over and over until you do it right. You're born into a family based on your karmic wealth. So if you were an asshole in the previous life, a not so good situation will be chosen for you on rentry.
I don't necessarily believe all this, but as a Unitarian Universalist (recovering Christian) I've taken to learning about alot of the different beliefs in the world and how picking just one because your told to, is ignorant.