What are the different types of indexes, what are the benefits of each?

Question

Unique – Guarantees unique values for the column(or set of columns) included in the index
Covering – Includes all of the columns that are used in a particular query (or set of queries), allowing the database to use only the index and not actually have to look at the table data to retrieve the results
Clustered – This is way in which the actual data is ordered on the disk, which means if a query uses the clustered index for looking up the values, it does not have to take the additional step of looking up the actual table row for any data not included in the index.