1) Game Badge.
A badge is an icon on your profile (and tied to your account) that represents the trading card sets you've collected or your participation in an event.
You can select any completed badge from your Badges page. Your featured badge will display at the top of your profile and in the mini profile you see when you hover over your avatar.
2) random emoticon item created for this game, that can use in chat or discussions;
3) random profile background item created for this game, that can use on your Steam Community profile;
4) chance to get a discount coupon for a different game or DLC;
5) 100XP, which may increase your Steam Level.

When your Steam Level increases, you get:
1) the limit on the size of your friends list will be increased by 5.
2) Once you reach Steam Level 10 you can pick and customize a showcase from the profile edit page. Each 10 Steam Levels earns you an extra showcase, i.e. two showcases at Level 20, three showcases at Level 30, etc.
3) Once you have received all of your card drops, you become eligible for a booster pack, which is a set of 3 trading cards that may include both basic and foil cards. Booster packs are granted randomly to eligible users as more badges are crafted by members of the community. Be sure, that you enter Steam each week to maintain eligibility.

Once eligible, your Steam Level increases your rate of receiving a booster pack drop:
Level 10: +20% increase in your drop rate
Level 20: +40% increase in your drop rate
Level 30: +60% increase in your drop rate
Level 40: +80% increase in your drop rate
Level 50: +100% increase in your drop rate (i.e. the rate has doubled)
Etc.