Emoticons (smileys) can be created by using different symbol combinations, the software then interprets the symbols and substitutes the correct smiley.
So a semi-colon : plus a closed bracket ) together make a 🙂
Semi-colon plus a P make 😛
Semi-colon plus a D make a grin 😀
There’s loads more but they might not all convert to a symbol, there’s some details on wikipedia:
http://en.wikipedia.org/wiki/List_of_emoticons