when we use dataset,datatable

Agree with Amit. Adding to what he said: when you assign dataset as datasource to a datagrid( or any other UI control), the first table( of the dataset) will act as datasource.

Hope that helps!

DataSet and DataTable are the key components in ADO.NET programming. This mean that DataTable represents an in memory representation of the database. We can load a single Table from the database into a DataTable and manipulate the data in memory. DataTable can be used as a DataSource and used it if we are going to fetch data in a single database table only. While DataSet on the other hand can define DataRelations - which define the relationship between DataTables, much like a foreign key relationship can be set up between tables in a database. DataSets, themselves DOES NOT Contain any Data. DataSets contain DataTables (which is where any data actually resides),DataRelations, etc, but no data.

SqlDataAdapter a = new SqlDataAdapter;
DataSet s = new DataSet();
a.Fill(s);
foreach (DataRow dr in s.Tables[0].Rows)
  {
    Console.WriteLine(dr[0].ToString());
  }

It really depends on the sort of data you're bringing back. Since a DataSet is (in effect) just a collection of DataTable objects, you can return multiple distinct sets of data into a single, and therefore more manageable, object.

we can use "DataGrid1.datasource=datatable" when we have to bind single datatable to a datagrid. we can also use datatable.defaultview to bind as a Datasource. It helps to filter & sort using dataview.

but when we have realtionship exists in datatables in Dataset, then we can bind Dataset to show related data.