Whelp, Googled it and it is exactly what I thought it was. Youtube for Business is just a regular Youtube account, but marketed for businesses. You need a certain pitch and explain certain things to businesses before they start using your platform. Plus, businesses have a very specific aim for which they will use their channel (reaching a target audience and marketing their product) . The only thing that seems different is that you can indicate that you will use Youtube for your business when you create an account. So to answer your questions:
Nothing. The whole 'Youtube for Business' page is just a pitch to lure businesses into using Youtube. Only difference is indicating that it is a business page. How this will affect the playing of ads or any adjustments to rules is not specified.
Whelp, owning a business. I don't think you need to be an officially registered one. If you want to start a Youtube account to sell your home made plushies, that's a business.