WoW Tokens are items that can be purchased by players for real money, and then sold to other players for in-game gold (World of Warcraft's currency).
You can buy WoW tokens directly from Blizzard for $20 USD, $25 AUD, €20 EUR, £15 GBP, ¥75 CNY, ₩22,000 KRW, or NT$500 TWD.
The gold price for a token is the same for ALL realms in a region. You'll find the current token price for each region at the top of this page. The prices change approximately every 5-10 minutes, and changes based on supply and demand.
A player may purchase a maximum of 10 tokens per week, and may only own 10 tokens at one time. This limit is shared across all characters on a Blizzard account.
You may use a WoW token to add 30 days of game time to your subscription. You may also use the token to add money to your Blizzard balance.
The WoW token will add the following balance to your Blizzard account: $15 USD, $17 AUD, €12.99 EUR or £9.99 GBP. China, Korea and Taiwan doesn't currently have this option available. For more information about your Blizzard balance, check here.
You may not buy a WoW token with money, then use it yourself. You may only sell it on the auction house.
You may not buy a WoW token with gold, and then re-sell it for gold. You may only add currency to your Blizzard balance, or use it for 30 days of game time.
If you don't currently have a subscription, you may login to World of Warcraft, and purchase a token from the character select screen.
For more information about using a WoW token, check here