ADO.NET has two types of objects: connection-based and content-based.

Connection-based objects: These are the data provider objects such as Connection, Command, DataReader, and DataAdapter. They allow you to connect to a database, execute SQL statements, move through a read-only result set, and fill a DataSet. The connection-based objects are specific to the type of data source, and are found in a provider-specific namespace (such as System.Data.SqlClient for the SQL Server provider).

Content-based objects: These objects are really just "packages" for data. They include the DataSet, DataColumn, DataRow, DataRelation, and several others. They are completely independent of the type of data source and are found in the System.Data namespace.