Ok, so once upon a time, someone thought that it'd be good to make homemade emoticons that anyone could upload and use on deviantART. There was already a system where you type :icon*insertusernamehere*: and get the icon of a user, so people used that. On dA, you can have multiple accounts, so people started making accounts and putting something specific as their avatar that could be used as an emoticon. They all use 'plz' at the end of their names. So, for example, :iconbieberplz: will get you Justin Bieber. There are thousands, so just a guess will probably get you something. You can also hover over a plz to see its name without the icon bit. Hope this helps and you have fun trying out the emotes!