Happy New Year My dear Friends 
It’s been a long this feature has been released however I just got an opportunity to check it out. This feature is released in SQL Server 2008 version. Sparse columns are normal columns which will store NULL values in a optimized storage which means no storage space is taken when you store NULL values to that column. Happy !! there is some overhead for storing non NULL values to this columns, it will take some more space for non NULL values.