Comma Before a Coordinating Conjunction
See FANBOYS for more info. (link will be hot later.)
When for, and, nor, but, or, yet, or so are used to connect two complete sentences there should be a comma before it. These words can be coordinating conjunctions, but they are not always, nor does a comma always need to be placed before them.
Joe was born in Hollywood, and he loves California.
We need a comma there mostly for organization and clarity. In some rare examples, one of which I hope to find for this blog, the meaning of the sentence can be totally changed by the absence of this comma.