Emoji are Unicode characters designed to represent pictograms and emoticons. The term itself, however, is most often used to refer to the pictograms and emoticons themselves. The word "Emoji" comes from the Japanese word "絵" (e, "picture") + "文字" (moji, "character"). Its resemblance to the word "emoticon" is purely coincidental.
Emoji as we know them originated as a proprietary system by a Japanese mobile phone operator NTT DoCoMo using a Unicode private-use block to encode their emoticons. While other mobile providers used many other systems for emoji, this system was the most compact implementation used at the time (and the closest to the current one).
The very first Emoji were actually Unicode Dingbats, of which, quite a few are now official Emoji. Emoji became a standardized system with its implementation in Unicode 6.0 in 2010 but it's only around 2014-2015 that their use exploded and became commonplace to the point that they became a cultural phenomenon with real world merchandise and even a movie about them.
