Non Cluster Index
A nonclustered index can be defined on a table or view with a clustered index or on a heap. Each index row in the nonclustered index contains the nonclustered key value and a row locator. This locator points to the data row in the clustered index or heap having the key value.
With a non clustered index there is a second list that has pointers to the physical rows. You can have many non clustered indexes, although each new index will increase the time it takes to write new records.
For more information refer these links
http://www.codeproject.com/Articles/234399/Database-performance-optimization-part-Indexing#NCIndexes
Refer this link on msdn for more information
https://msdn.microsoft.com/en-us/library/ms175049.aspx