Steam Wallet

The Steam Wallet is a feature Valve has implemented into the Steam content delivery system for the purpose of micro-transactions. The Steam Wallet beta was released on September 10, 2010, and officially released on September 30, 2010. The Steam Wallet is basically a virtual bank which can be used on Steam to purchase games, downloadable content (DLC), and other steam based content. For example, you can use your Steam Wallet funds to buy the Bionic Beanie from the Robot Enrichment Store.

Usage
With Steam Wallet, you can add value to your Steam account using any of the payment methods supported by Steam. Funds in your Steam Wallet may then be used to buy any item available in the Steam store on an in-game store. You can load up your Steam Wallet ahead of time, or wait until checkout. Also, different payment methods can be combined to make a single purchase.

Once you've added funds to your wallet, that money will be used by default for subsequent Steam store purchases. But if you'd prefer to reserve your wallet balance for in-game purchases of Safety Glasses, BIT.TRIP Flag, and Ninja Skins, no problem -- simply select another payment method during checkout. Your wallet funds will be there the next time you're ready to shop at the Robot Enrichment Store, or any other in-game store.