For those of you not aware of what a Stickers App is here is a great explanation from iMore.com
Apple has made it possible for you to make your messages a lot more interesting with the help of apps and stickers in iMessages. You can use stickers as stand-alone images, or slap them right on top of someone else’s chat bubble (or sticker, or picture). Apps integrate information so you can, say, book a table at a restaurant or share movie times with others. Your friends can also change dates and times in some instances so everyone can collaborate together.