HINTS  are nothing but the comments used in a SQL statement to pass instructions to the Oracle optimizer.The optimizer uses these hints as suggestions for choosing an execution plan for the statement.

A statement block can have only one comment containing hints, and that comment must follow the SELECT, UPDATE, INSERT, or DELETE keyword..

Hints can be categorized as follows:

• Hints for Optimization Approaches and Goals (eg: ALL_ROWS)
• Hints for Access Paths,
• Hints for Query Transformations (eg:NO_EXPAND ) 
• Hints for Join Orders,
• Hints for Join Operations,
• Hints for Parallel Execution,
• Additional Hints (eg:NOCACHE ,APPEND)