Facebook Stickers which were previously introduced in Android/iOS and Java apps are now available in Facebook Web and you can directly insert them to your chat conversations by clicking on the Smiley icon (which you used to insert smileys like 🙂 and 🙁
2 set of stickers are already available to everyone, but you can easily install and activate more from the Facebook Sticker Store which now offers many different sets of stickers. All the various sets of stickers are free for now, but the inclusion of a pricing option clearly shows up that Facebook might introduce paid sticker sets as well.
Facebook has been trying hard to bring in new revenue sources and this could be another way to attract some interested folks who want to be different than the rest.