WinForms is the informal term given to the graphical application programming interface (API) included as a part of Microsoft's .NET Framework, providing access to the native Microsoft Windows interface elements by wrapping the existing Windows API in managed code.

Definition (Wikipedia):

Windows Forms (WinForms) is the name given to the graphical application programming interface (API) included as a part of Microsoft .NET Framework, providing access to native Microsoft Windows interface elements by wrapping the extant Windows API in managed code.

From MSDN:

As forms are the base unit of your application, it is essential that you give some thought to their function and design. A form is ultimately a blank slate that you, as a developer, enhance with controls to create a user interface and with code to manipulate data. To that end, Visual Studio provides you with an integrated development environment (IDE) to aid in writing code, as well as a rich control set written with the .NET Framework. By complementing the functionality of these controls with your code, you can easily and quickly develop the solutions you need.

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