An emoji, or emoticon, is a unicode character that is commonly displayed on devices as colorful symbols or faces.
These can range from faces, to symbols, to animals, to plants, etcetera.
Not every device can show emojis. If you see a blank box, a question mark in a box or anything similar (�), that means your device is incapable of displaying that character.
The word "Emoji" comes from the Japanese word "e" ("picture") and "moji" ("character").