Emoji are picture characters originally used on mobile phones in Japan which are now used worldwide. In Japanese the word emoji means "picture" (e) + "character" (moji).
Emoji are mainly used to easily and quickly express emotions and activities within the character limits of text messages.
An emoji is encoded as a single character with the Unicode standard and is displayed properly across many platforms. Nevertheless, a same emoji may have different look & feels according to mobile platforms.
Today emoji are also quite popular in chat, on Twitter and with some email applications like Gmail. There are more than 700 different emoji.
Strictly speaking emoji differ from emoticons, the common "emo" root being pure chance.
Emoji are sometimes used in digital marketing : see emoji marketing.
Emoji by platform :
image credit Emojipedia