Emoticon: A symbol used in text-based online communication to indicate the mood of the author or the intended tone of the communication. They are composed of symbols from the standard English keyboard, often depicting a human face, turned on its side, exhibiting facial cues about mood.
Also known as a smiley.
Emoticons date back to the early 1980’s, before graphical browsers became available, and are still used to contextualize informal written information in email, via the web, and in telephone based text-messaging (SMS).
A brief history of smileys is offered here: http://www.nerdtimes.com/emoticons/
A partial list of commonly used emoticons is also available at that same site (http://www.nerdtimes.com/smileys.htm) – a larger dictionary is also available there for those who really need to express themselves.
Emoticons are useful in any informal, text-based communication. The emotional bandwidth, so to speak, of such media is very narrow. Without such emotional context clues, written messages can easily be misinterpreted.