Answer
Clustered Index: Defines the physical order of rows in the table. Implemented as a B-Tree with root, intermediate, and leaf nodes. The leaf level contains actual data rows.
Non-Clustered Index: Also a B-Tree, but leaf nodes store index keys and row locators (RID or clustered key).
Performance Impact: Clustered indexes help range queries, while non-clustered indexes help quick lookups. Poor clustered key choice can make indexes large and slow.