😊for humor, laughter, friendliness, occasionally sarcasm
☹️for sadness, anger, or upset
😉(ha ha only serious); also known as semi-smiley or winkey face
😕for confusion or WTF
😏for sarcasm... as always
(These may become more comprehensible if you tilt your head sideways, to the left.)
The first 2 listed are by far the most frequently encountered. Hyphenless forms of them are common on CompuServe, GEnie, and BIX; see also bixie. On USENET, 'smiley' is often used as a generic term synonymous with emoticon, as well as specifically for the happy-face emoticon.
It appears that the emoticon was invented by one Scott Fahlman on the CMU bboard systems around 1980. He later wrote: "I wish I had saved the original post, or at least recorded the date for posterity, but I had no idea that I was starting something that would soon pollute all the world's communication channels."
[GLS confirms that he remembers this original posting].
Note for the newbie: Overuse of the smiley is a mark of loserhood! More than one per paragraph is a fairly sure sign that you've gone over the line.