Essentially, you have a Battle.net Balance made up of points. Adding value to your Battle.net Balance is executed through charging up. You can charge up your Battle.net Balance using debit or credit cards, or sales from Diablo III auctions. Be aware, though, that you can't convert Battle.net Balance back into cash; once you put money there, it stays there. (There is an exception in some regions using PayPal, but details are scarce on that right now.)
Some of the Battle.net details are a little fuzzy right now because Blizzard is forced to deal with a lot of regional-specific laws. For example, in some regions, it'll have to empty value from a Battle.net Balance that hasn't been accessed in three years. Which regions? We don't know yet.
The most exciting bit of this news, however, is that you'll be able to purchase World of Warcraft prepaid game time using your Battle.net Balance. So if you're pretty good at the Diablo III Auction House, you might be able to kiss your subscription fee goodbye.