What is the difference between clustering and non-clustering indexes?

A clustering index determines the physical order of rows in a table; there can be only one clustering index per table, and the data is stored on disk in that order. A non-clustering (or secondary) index is a separate structure that contains key values and pointers to the actual rows. Multiple non-clustering indexes can exist. Clustering indexes are useful when range queries on the key are common, as the data is contiguous. Non-clustering indexes are better for lookups on columns that are not the primary key, but they require an extra lookup to fetch the row data.