Apple released iOS 10 in September 2016, along with a new iMessage app store featuring stickers and mini apps. How do you get to this new store you ask?
First, open iMessage, then press the grey and white arrow to the left of your text input box.
Second, press the third icon from the left for the new iMessage App Store.
Lastly, press the icon on the bottom left of the screen represented by four gray ovals.