As the song goes, "WoW® Tokens make the world go around, the world go around, the world go around..." but obviously by "world" they mean "Blizzard-iverse" because you can now convert the WoW® Tokens you buy with gold in World of Warcraft to a digital currency called Battle.net Balance which you can use to purchase Blizzard games as well as in-game items for Hearthstone, Overwatch and the like.
WoW® Tokens are in-game items which you buy using gold through the auction house system or via real money in World of Warcraft and can either sell for gold or redeem for 30 days of game time or $15 of Battle.net Balance. If you're familiar with EVE's PLEX system it's kind of like that.
Battle.net Balance is a Blizzard-specific form of digital currency where you can't cash out, so you can't convert your WoW® millions into actual spending money at the corner shop, but you can use it to buy various Blizzard doodads related to Blizzard games. The support site lists World of Warcraft, StarCraft 2, Diablo 3, Battle.net, Hearthstone, Heroes of the Storm, Overwatch and BlizzCon as relevant to the information so presumably all will make use of the system in some fashion.
As per the website:
Use Battle.net balance to buy World of Warcraft realm transfers and other paid services, as well as digital versions of Blizzard games like Diablo III and StarCraft II.
You can also use Battle.net balance to buy items in games like World of Warcraft and Hearthstone.
Because I'm fun and I read the restrictions there are a few bits to be aware of:
"You can't use Battle.net Balance to purchase items from the Blizzard Gear Store, in-game products for Hearthstone on mobile devices, or World of Warcraft subscriptions — though you can buy game time in the Battle.net Shop."
There are also balance limits if you don't use the Battle.net Authenticator security system and you can't make a partial payment using Battle.net Balance. It's all or nothing, so this is presumably to get people to chuck bits of real cash into the system to top up balances and make them usable. I've just logged in and under the bit where I can add real money it tells me "Your balance must be at least GBP 9.00 and cannot exceed GBP 200.00".
Oh, and if you do top up your balance it might take several days to come through:
"Note: To ensure the security of every Battle.net Balance transaction, there may be a delay before Battle.net Balance becomes available for use. Credit card purchases may take up to 3 days to process. Direct debit users can use PayPal as an alternative method of payment."
The limits and restrictions are here if you want to read them.
I don't play WoW® so I've been training my caffeinated gaze on the forums for responses. Looks like so many people are trying to sink their excess gold into Tokens that the Token well is running dry and auction houses are selling out almost instantly. There are also some interesting discussions (in amongst the insults, the armchair economists, and the telling other people to learn how to use the internet/unsubscribe from WoW®/ask for more pocket money) about the worth of a Token right now and their purpose with regard to World of Warcraft and game time now.