Anti Join / Semi Join

ANTI JOIN : http://wiki.oracleclub.com/display/CORE/Anti+Join 

SEMI JOIN : http://wiki.oracleclub.com/display/CORE/Semi+Join 

ppt :

Speeding Up Queries with Semi-Joins and Anti-Joins

html : http://www.dbspecialists.com/files/presentations/semijoins.html