Originally invented for use in Japanese cellphone text messages, emoji are small pictures that can be used to represent emotions, similar to emoticons, or just about anything else. The same emoji will appear differently across different platforms and typefaces, much like a typographical font.
Emoji became extremely popular outside of Japan following their inclusion in Apple's popular iPhone smartphone. They were later added to Android and are now increasingly found on websites and other platforms as well. Because of their origins in Japan, many emoji are specific to Japanese culture, such as a bowing businessman, a Japanese flag, and Japanese foods such as onigiri and kushiyaki.
Many people are surprised to learn that the word "emoji" is actually not related to the English word "emotion," the way the similar "emoticon" is. Rather it is simply a Japanese word meaning "picture character" (絵文字).
The first emoji were created by Japanese designer Shigetaka Kurita in 1998, intended for use in NTT Docomo's iMode phone platform. The images soon spread to other Japanese mobile carriers. Emoji remained confined to Japan for more than a decade until 2010, when 722 existing emoji were ported into Unicode 6.0, at the request of Apple and Google. This allowed emoji to go global. Some of the emoji were mapped onto preexisting unicode entities, while the rest were created as new entities. Since that time, hundreds of new emoji have been added to Unicode, many of which did not previously exist in Japan.
Today, there are enough emoji that people have started treating them as their own language. Most notably, somebody has translated the entire novel Moby Dick into an emoji version, entitled Emoji Dick.
Some examples of emoji (these will look different on different devices):