There is a book called the DSM (Diagnostic and Statistical Manual) the American Psychiatric Association puts out that gives all current psych diagnoses, the criteria to meet them, etc. They don't update it all that often, so when they do it is a huge deal.
The next edition will be out in 2013. Right now they are accepting comments from healthcare providers, psych patients and the general public about the proposed changes.
I'm so grateful to have had a chance to give input, as they actually intend to shove mixed episodes (depression and mania together) into mania, which takes away the power of describing this, because it is not a type of mania, it is far worse than mania.
So please look around and give them feedback.