System.data contains basic objects. These objects are used for accessing and storing relational data. Each of these is independent of the type of data source and the way we connect to it.
These objects are:
DataSet
DataTable
DataRelation.
System.Data.OleDB objects are used to connect to a data source via an OLE-DB provider.
These objects have the same properties, methods, and events as the SqlClient equivalents.
A few of the object providers are:
OleDbConnection
OleDbCommand
System.Data.SqlClient objects are used to connect to a data source via the Tabular Data Stream (TDS) interface of only Microsoft SQL Server.
The intermediate layers required by an OLE-DB connection are removed in this.
This provides better performance.
System.XML contains the basic objects required to create, read, store, write, and manipulate XML documents according to W3C recommendations.