With every game you play, win or lose, you will be rewarded with XP (eXperience Points). Winning will give you more points than losing, but by playing you always gather points. When you reach a certain number of points, your level will go up. There is an XP bar displayed in your pop up stats screen so you can monitor your progress. After every game the results screen will tell you the amount of XP you will receive.
Note: You have to complete at least 80% of the game to earn XP.
In-game achievements are rewarded with medals. These medals will appear in your in-game medals tab , allowing you to see all the medals you've gathered yourself or see other players' medals by pressing on their medals in their stats.