craftgrrl | Registering a business

Hi,
If this isn't the right community to post this question in please refer me to another if you know one.
I am interested in actually selling some of my crafts at craft fairs or in small stores or even online. Do you actually have to register as a business to do this? I am currently residing in California. I have tried to search all sorts of government sites and craft start up sites, but none tell me why you would have to register a business. Do all of you who actually sell online and such have a registered business?
Thanks for your help!