As of version 1.2.4 there are sixteen (16) in-app purchases. (for general information regarding in-app purchases, refer to iTunes Store: About In-App Purchases)
They are things that can be bought in the Shop to enhance your game. Once you bought them, you have the option to enable or disable it. You can use your purchase on any device that uses the same apple id that used to buy your item.
Hats will occasionally drop items.
Multiple hats can be enabled at once, however, drops will only be received from one hat, as indicated by the crosshair (player graphic).