One of the new messaging features introduced in iOS 10 is Stickers. These are static or animated images that you can attach to text bubbles in the Messages app. Stickers are available in packs on the App Store. You can access sticker packs by either downloading an app that features its own stickers or by downloading specifically created sticker packs from the iMessage App Store.
The access the iMessage App Store, open your Messages app and tap the grey arrow next to the iMessage text box. Tap the App Store icon to open your app drawer, then tap the four grey dots in the lower left hand corner. Finally, tap the blue + sign labeled "Store" and you will not be able to access all the premium and free available sticker packs.