ADO.Net

ADO.NET : ADO.NET is a set of classes provided by .NET Framework used to communicate and perform Database operation with relational database such as SQL Server , Oracle, My SQL.

Lets see some other definitions of ADO.NET:

• ADO.NET is a set of computer software components that can be used by programmers to access data and data services. It is a part of the base class library that is included with the Microsoft .NET Framework. It is commonly used by programmers to access and modify data stored in relational database systems, though it can also be used to access data in non-relational sources. ADO.NET is sometimes considered an evolution of ActiveX Data Objects (ADO) technology, but was changed so extensively that it can be considered an entirely new product.
  (Source wikipedia)
  
• ADO.NET provides consistent access to data sources such as Microsoft SQL Server, as well as data sources exposed through OLE DB and XML. Data-sharing consumer applications can use ADO.NET to connect to these data sources and retrieve, manipulate, and update data.
  (Source MSDN)

So, what all the classes are present in ADO.NET? 

SQLConnection - used to connect with MS SQL DataBase.
SQLCommand - Used to perform set of SQL Query like update select delete the records.
SQLDataReader - Used to read records from database with connected architecture with Database.
SQLDataAdapter - Used with Dataset to do Database operation in disconnected environment.