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Some basic examples:
:-) is a smile :-( is a frown :-P has a lolling tongue ;-) is winking B-) or 8-) has sunglasses :-o surprise :-7 tongue in cheek
...there are a lot of possibilities, people are very good at interpreting pictures as faces. See Ascii Art, maybe [Electronic communications]?, Emote?, or Emoticon?.
Note: "Frownies" :-( were trademarked by [Despair Inc] in [US Patent Registration Number: 2347676]