The best I can gather from the documentation is that "signed" or "unsigned" refers to the plus or minus sign a person would traditionally use to indicate a positive or negative value. So an UNSIGNED numeric value can never be negative.

Therefore, if you have a column that you want to contain a numeric value, but never hold a negative value, I guess you could apply the UNSIGNED modifier to the data type.

http://dev.mysql.com/doc/refman/5.0/...-overview.html