ADO.NET

In .NET (C#, ASP.NET, Windows Forms, WPF), if you need to work with a database (SQL Server, Oracle, Access etc), you need some classes. These classes are defined in the .NET Framework and this group of classes (namespaces, dlls) is called ADO.NET. You can also call it API (Application Programming Interface). 

.NET Framework Class Library has several APIs to do different work. For example, ADO.NET is to work with databases. GDI+ is to work with graphics. Remoting is to transfer data between two computer and so on.