Index types in SQL Server

Clustered Index

Only 1 allowed per table physically rearranges the data in the table to confirm to the index constraints for use on columns that are frequently searched for ranges of data for use on columns with low selectivity.

Non-Clustered Index

Up to 249 allowed per table creates a separate list of key values with pointers to the location of the data in the data pages For use on columns that are searched for single values 

A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table can have only one clustered index. The leaf nodes of a clustered index contain the data pages. A non-clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.