It's a reading card.

If you get enough you can swap them for a coupon on steam

HOW IT WORKS: TRADING CARDS

What are trading cards?
Steam Trading Cards are virtual cards earned by buying and playing games on Steam. Trading Cards can also be crafted into game badges that grant community rewards.
Get an introduction to trading cards.
How do I get Summer Getaway trading cards?
Normally you would start a trading card set by playing a game. Since in this case the sale itself is the game, here are the ways you can get Summer Getaway cards:
Craft a game badge
Starting July 3rd, crafting a game badge earned you a "mystery card" rather than a coupon. These mystery cards are now Summer Getaway cards. So if you crafted a badge in the last week, you're already ahead of the game. During the sale, each game badge you craft will get you a Summer Getaway trading card.
View the participating trading card games I own.
Make a purchase
As an additional reward for purchasing, for every $10 USD spent during the sale you'll get a Summer Getaway trading card. To view your progress to the next card, select the "How do I get more cards?" link from your Summer Getaway badge page. Note: applies to purchases made on the Steam store (including gifts), excluding pre-purchases and in-game purchases.
Vote in the Community's Choice
If you're Steam Level 5 and above, you'll get a card every third time you vote for the Community's Choice Flash Sale. If you reach Steam Level 5 before the end of the sale, you'll be granted the cards you earned by voting.
Trade for them
That's why they're called trading cards! Find friends and make trades. Your Summer Getaway badge page shows which friends have the cards you need.
Buy them from the Community Market
Sell the cards you don't need and buy the ones you do with other Steam Community members. If you aren't interested in badge-crafting, you can also turn the cards into Steam Wallet funds to buy more games.
View the Community Market.
CRAFT THE BADGE AND EARN REWARDS

How do I craft the Summer Getaway badge?
Once you've collected the entire set of cards, you can craft them into the Summer Getaway badge which you can then display on your profile. From your Badges page, select a "ready to craft" badge to view its details, and then click on the blue "Craft a Badge" button.
View your Summer Getaway badge page.
What do I get when I craft a badge?
Crafting the Summer Getaway badge (and other game badges) earns you tradable items like emoticons to use in chats or discussions and backgrounds to use on your profile. You can collect the set of cards five times to level up your Summer Getaway badge and earn more items. Each time you craft a badge you also earn XP which progresses you towards the next Steam Level.
What is my Steam Level?
Your Steam Level is a summary of your badges and participation in Steam events. Each badge earned increases your XP, and earning XP progresses you towards the next Steam Level.
straight off Steam its self

basically you can trade them like TF2 items or craft them into privages for your profile or something i depends what card you get

SupremeThe_BOSS wrote HOW IT WORKS: TRADING CARDS

What are trading cards?
Steam Trading Cards are virtual cards earned by buying and playing games on Steam. Trading Cards can also be crafted into game badges that grant community rewards.
Get an introduction to trading cards.
How do I get Summer Getaway trading cards?
Normally you would start a trading card set by playing a game. Since in this case the sale itself is the game, here are the ways you can get Summer Getaway cards:
Craft a game badge
Starting July 3rd, crafting a game badge earned you a "mystery card" rather than a coupon. These mystery cards are now Summer Getaway cards. So if you crafted a badge in the last week, you're already ahead of the game. During the sale, each game badge you craft will get you a Summer Getaway trading card.
View the participating trading card games I own.
Make a purchase
As an additional reward for purchasing, for every $10 USD spent during the sale you'll get a Summer Getaway trading card. To view your progress to the next card, select the "How do I get more cards?" link from your Summer Getaway badge page. Note: applies to purchases made on the Steam store (including gifts), excluding pre-purchases and in-game purchases.
Vote in the Community's Choice
If you're Steam Level 5 and above, you'll get a card every third time you vote for the Community's Choice Flash Sale. If you reach Steam Level 5 before the end of the sale, you'll be granted the cards you earned by voting.
Trade for them
That's why they're called trading cards! Find friends and make trades. Your Summer Getaway badge page shows which friends have the cards you need.
Buy them from the Community Market
Sell the cards you don't need and buy the ones you do with other Steam Community members. If you aren't interested in badge-crafting, you can also turn the cards into Steam Wallet funds to buy more games.
View the Community Market.
CRAFT THE BADGE AND EARN REWARDS

How do I craft the Summer Getaway badge?
Once you've collected the entire set of cards, you can craft them into the Summer Getaway badge which you can then display on your profile. From your Badges page, select a "ready to craft" badge to view its details, and then click on the blue "Craft a Badge" button.
View your Summer Getaway badge page.
What do I get when I craft a badge?
Crafting the Summer Getaway badge (and other game badges) earns you tradable items like emoticons to use in chats or discussions and backgrounds to use on your profile. You can collect the set of cards five times to level up your Summer Getaway badge and earn more items. Each time you craft a badge you also earn XP which progresses you towards the next Steam Level.
What is my Steam Level?
Your Steam Level is a summary of your badges and participation in Steam events. Each badge earned increases your XP, and earning XP progresses you towards the next Steam Level.
straight off Steam its self

basically you can trade them like TF2 items or craft them into privages for your profile or something i depends what card you get

Cheers mate, really helped (Repped you)