OK, keys can have as many duplicate values as you want. Unique keys can't - they cant have any dupes. Primary keys are used usually as a unique auto-increment key, such as an "id". (0, 1, 2, 3...) There must be a primary key for every table in a database.
for more info, goto:
http://dev.mysql.com/doc/mysql/en/CREATE_TABLE.html
