Steam Gift cards work just like a gift certificate that can be redeemed on Steam for the purchase of games, software, wallet credit, and any other item you can purchase on Steam. You can find Steam Gift Cards at plenty of retail stores across the world in a variety of denominations.
Important Changes By Steam:
Each Steam User Account is associated with a specific currency, which you can see in the top right corner of your Steam Client. Going forward, you may encounter an error if you try to activate a Steam Wallet Code issued in a different currency than the one of your Steam User Account.