SET('value1','value2',...) [CHARACTER SET charset_name] [COLLATE
collation_name]

A set. A string object that can have zero or more values, each of which
must be chosen from the list of values 'value1', 'value2', ... A SET
column can have a maximum of 64 members. SET values are represented
internally as integers.

URL: http://dev.mysql.com/doc/refman/5.5/en/string-type-overview.html

