What is the difference between clustered and nonclustered indexes?

A clustered index contains the table data at its leaf level and determines the table's clustered storage order. A nonclustered index is a separate structure containing index keys and a locator to the corresponding row.

A table can have only one clustered index but multiple nonclustered indexes. Neither type is universally faster; performance depends on the query, key order, selectivity, included columns, data distribution, and workload.