LINQ to Dataset provides the easier and faster approach to query over data cached in a DataSet object.
LINQ to DataSet can also be used to query over data that has been consolidated from one or more data sources.
For example, querying data on actual data and intermediate cached data source in financial web application, such scenarios required to create financial reports.
Following diagram explain relationship between LINQ to Dataset to ADO.NET 2.0 and Database.
More details ..http://codechef4u.com/post/2016/03/27/linq-to-dataset