It's about putting emotions on text. Twenty-five years ago, Carnegie Mellon University professor Scott E. Fahlman was the first to use keystrokes to put the "smiley face" in an e-mail. That started the deluge of emotion icons, popularly called "emoticons," that have added a great dimension to the basic fabric of communication.
A quote from the article at CNN.com:
Fahlman posted the emoticon in a message to an online electronic bulletin board at 11:44 a.m. on September 19, 1982, during a discussion about the limits of online humor and how to denote comments meant to be taken lightly.
"I propose the following character sequence for joke markers: :-)," wrote Fahlman. "Read it sideways."
And since that day, the emoticon has become an essential part of most messenger software, replacing the textual combinations to more appealing images that convey the emotion. The emoticon was a great breakthrough in conveying the general feeling behind "texted" communication.
Emoticons and short text acronyms are one of the defining parameters of the current tech generation, but these often invite the ire of language lovers (Wired).