What does this Deterministic finite automaton do?

nighthound

1. The problem statement, all variables and given/known data
Can someone explain the condition that DFA(Deterministic finite automaton) take as an accepting state and rejecting state?


2. Relevant equations
http://img641.imageshack.us/img641/8387/dfao.jpg


3. The attempt at a solution
I have tried analyzing through it, but it seems pretty random to me.
It accepts: empty,ab, ba, abab, baba, aabb, bbaa, bbb, aaa, bbaaab, aabbba,....
Rejects: a, aa, aab, aba, b, baa, bb, bba, bbaaa, ....