Let me clear up a common misconception. When most people say writing in code, what they really mean is writing using a cipher. A code is when a symbol, word, or phrase is used to stand for a different word, phrase, or sentence. A cipher is were each letter in a word is represented with a different letter or symbol.
The dash is a punctuation mark that is similar to a hyphen or minus sign, but differs from both of these symbols primarily in length and function.
The difference between a dash and a minus sign. A reader asked about the difference between a minus symbol and an N-dash (thanks Tem). I had to do a bit of homework on this. Unicode (the standard for electronically encoding text information) has separate codes for the minus sign, hyphen and N-dash.