Explain the difference between clustered and non-clustered indexes.

Clustered Index:
• Physical order of data matches index order
• One per table (usually PRIMARY KEY)
• Data pages stored in order of key values
• Faster for range queries

Non-clustered Index:
• Separate structure pointing to data rows
• Multiple allowed per table
• Contains pointers to actual data
• Additional lookup required

In MySQL InnoDB, PRIMARY KEY is clustered, others are non-clustered.