Valve has announced that Steam Trading Cards are coming out of beta on Wednesday, and that this is "a step towards an event we think everyone can be excited about..."
We're assuming that the event in question is Valve's annual Summer Sale, where lots of games get heavily discounted. If this is the case, now would be a good time to top up that Steam wallet with funds.
The Steam Trading Cards are virtual cards that players receive playing games on Valve's platform, and completed sets can be turned into badges or traded for community items. Players will have to trade with other players, as only half of the cards will be dropped during gameplay.
Full sets can be crafted into game badges, and these can earn players "marketable" items such as emoticons, backgrounds, and perhaps even discount coupons. Badges can be leveled up, and non-tradebale items can also be earned. More details can be found here.