A dense index stores an index entry for every search key value in the file, whereas a sparse index stores an entry for only some search key values.   

A sparse index stores an entry for every record in the file, while a dense index stores only one entry per block.  

Dense indexes are used exclusively for non-clustered indexes, while sparse indexes are used exclusively for clustering indexes.

Sparse indexes are unordered, whereas dense indexes are always maintained in sorted order.