Is a Secondary Index in MySQL the Same Thing As a Non-Clustered Index in SQL Server?
http://dev.mysql.com/doc/refman/5.0/en/innodb-index-types.html


  In InnoDB, the records in nonclustered indexes (also called secondary
  indexes) contain the primary key columns for the row that are not in
  the secondary index. InnoDB uses this primary key value to search for
  the row in the clustered index. If the primary key is long, the
  secondary indexes use more space, so it is advantageous to have a
  short primary key.


Could have just searched for this.