From the New York Times:
“The first line of my obituary is going to mention the smiley face,” says Scott Fahlman, who would rather be remembered for his research into artificial intelligence. But like it or not, Fahlman has become famous for three keystrokes. In 1982, as a young professor at Carnegie Mellon University, he realized the need for a symbol to temper the bickering that plagued online forums. The Internet was just a baby then, and yet already flame wars raged. Fahlman decided that a smiley face could be useful as a “joke marker” (as he called it) to take the sting out of mocking statements or pranks. And so he hunted around the keyboard for a way to make the face. “But what do you use for eyes?” he wondered. Once he found the colon, the rest was easy. He dashed his suggestion off to friends. “I didn’t even proofread the message,” he says.