What is difference between primary index, secondary index, unique index and normal index?
Primary index 

A primary index is an index on a set of fields that includes
the unique primary key for the field and is guaranteed not to contain duplicates.
Also Called a Clustered index. 
eg. Employee ID can be Example of it.

Secondary index

A Secondary index is an index that is not a primary index and may have duplicates.
eg. Employee name can be example of it. Because Employee name can have similar values.

Dense Index

Index record appears for every search­ key value in the file.
Dense indexes point directly to individual records.

Sparse index

contains index records for only some search ­key values.
Applicable when records are sequentially ordered on search ­key.
Just as with book indexes, sparse database indexes don’t point to individual records, but to ‘pages'