N/Direct is a site for developers seeking information or answers about interoperability between .NET managed code and native unmanaged code. The site was created by me, Mattias, initially to host an unofficial FAQ for the Microsoft .NET Interop newsgroup microsoft.public.dotnet.framework.interop.
The name N/Direct was taken from what appears to have been a working name for what is now known as P/Invoke (Platform Invocation Services). The name can still be found in certain parts of the .NET Framework and Microsoft's Shared-Source release of the CLI (Rotor). Compare with J/Direct, the technology used for the same purpose in Visual J++.
Interop(erability) has become an overloaded term used in many different contexts. If you're looking for information about interoperability between Web Services, you've come to the wrong place. Try the Web Services Interoperability Organization (WS-I) instead.
This site is only about interoperating between managed and unmanaged code in the .NET environment. Even that is a pretty broad topic.
While COM+ and Enterprise Services is somewhat related to COM interop, I've decided not to cover that here. It's such a big topic in itself, and really deserves its own site. If you're interested in Enterprise Services, I suggest you look at GotDotNet or get the book COM and .NET Component Services by Juval Löwy.