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

An enumeration. A string object that can have only one value, chosen
from the list of values 'value1', 'value2', ..., NULL or the special ''
error value. An ENUM column can have a maximum of 65,535 distinct
values. ENUM values are represented internally as integers.

URL: http://dev.mysql.com/doc/refman/5.1/en/string-type-overview.html

