.NET/Link is a toolkit that integrates Mathematica and .NET. It allows you to call .NET from Mathematica and call Mathematica from .NET.
.NET/Link allow Mathematica programmers to load arbitrary .NET types into Mathematica, create objects of these types, call methods and properties, and so on. You can, in effect, "script" .NET from Mathematica. This powerful capability literally makes all of .NET accessible directly from the Mathematica language and allows exciting new categories of Mathematica programs to be created, incuding creating user interfaces, controlling COM objects like Microsoft Office, interacting directly with the Windows operating system, and many others.
Calling .NET from Mathematica is documented in Part 1 of the .NET/Link User Guide. The help file you are currently reading will not be of much value, as it describes the .NET/Link API as seen by .NET programmers, not Mathematica programmers. It can still be useful for Mathematica programmers who want to use one of the .NET/Link classes that are designed to be used from Mathematica, such as the MathPictureBox class.
.NET/Link allows programmers writing in any .NET language, such as C# or Visual Basic .NET, to launch Mathematica and use it as a computational engine. This help file describes the .NET/Link API that .NET programmers will use to interact with Mathematica.
A good place to start is the IKernelLink interface. You will find a simple example program there. IKernelLink is the link interface that most programmers will use.
You should also look at the MathKernel class. This component provides a very high-level interface for interacting with Mathematica. It is especially intended for use in visual programming environments, as it is highly configurable via properties. It is especially useful for programs that want to capture not just the result of a computation, but also messages, Print output, or graphics generated as side effects of the computation.
There are example programs in the .NET/Link examples directory: <Mathematica dir>\SystemFiles\Links\NETLink\Examples\Part2.
There is more documentation for .NET programmers in the .NET/Link User Guide. Part 2 of the User Guide describes writing .NET programs that call Mathematica, and it treats various topics in .NET/Link programming. The present help file is the API-level documentation.
.NET/Link consists of a single assembly, Wolfram.NETLink.dll. The Mathematica installer creates a special registry key that allows the Wolfram.NETLink assembly to automatically appear in the Visual Studio "Add Reference" dialog.
The latest version of .NET/Link can be freely downloaded from the .NET/Link home page.
Wolfram Research home page | .NET/Link home page | Wolfram.NETLink namespace | IKernelLink interface | MathKernel class