Define candidate key, alternate key, composite key.

A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys. A key formed by combining at least two or more columns is called composite key.

//link for Composite key understand

http://weblogs.sqlteam.com/jeffs/archive/2007/08/23/composite_primary_keys.aspx