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First it is important to describe to you what Steam gift cards really are. They can be considered basically presents which you can give to your friends, members of your Steam family etc. Each Steam wallet gift card has a target amount of money in it (in case of this card, it is 20 USD) which is going to be send to a Steam Wallet of a target person. With the money on the Steam wallet you can buy various games, apps and all the other things provided by Steam. Use the money wisely.
To redeem the Steam gift card you do not need a lot of time. You just need to go on your Steam account then enter your wallet and choose the option "Redeem a Steam Gift Card or Wallet Code". After that you need to enter code from the card and that's all. Funds are going to be transferred to your wallet. Not only it is easy but everything (buying the card, giving it to someone, redeeming it) can be done online, with a few clicks of your mouse. What else do you need? The question which might still remain though is probably, what can you buy with 20 USD on your Steam wallet? Well, a lot of games basically. 20 USD is enough money to provide you with really interesting games.
No expiration date.