WoW Tokens are a World of Warcraft item, rolled out in 2015, that players can buy and sell for either in-game gold or real money, and exchange for 30 days of World of Warcraft game time. The idea is to give gold-rich players something practical to spend their wealth on, and to enable others to purchase gold without having to go through a third-party gold seller. Today Blizzard announced that it is broadening the scope of Tokens to include "Battle.net Balance," which can be used to purchase services like WoW character transfers, pets, and mounts, and also in-game items from other support Blizzard games.
The process of buying a token remains unchanged, but once it's in your inventory, right-clicking it will give you the option to either add the usual 30 days of World of Warcraft time to your account, or to convert it to $15 (or the regional equivalent) on your Battle.net account. Tokens don't expire, so you can buy one and then sit on for as long as you like, and the new deal applies to any tokens that are already in your inventory, as well as newly-purchased ones.
There are some limits on how you can use your Battle.net Balance. Funds added to it are not refundable, non-transferable, and cannot be converted into real money. It can't be used to purchase items from the Blizzard Gear Store, and you can't use your balance to make partial payments, or mix it with other forms of payment. The Balance Restrictions page says that players who don't use a Battle.net authenticator will be "limited to a specific balance," and while the amount in question isn't provided, selecting the option to add funds to my account (with an authenticator) gives me a maximum of $350.
The support page lists "relevant games" as World of Warcraft, StarCraft 2, Diablo 3, Hearthstone, Heroes of the Storm, and Overwatch—pretty much every current Blizzard game, in other words. That's the good news: The bad news, as noted by Blizzard Watch and in the comments following the announcement, is that the supply of WoW Tokens ran dry almost immediately after the feature went live.