What is OWIN? How does it relate to Web?

.NET is Microsoft Specific which is also featuring in some Open source projects. Believe me it is true, read here:

https://opensource.com/business/14/11/microsoft-dot-net-empower-open-source-communities

'Microsoft is providing the full .NET server stack in open source, including ASP.NET, the compiler, the .NET Core Runtime, Framework and libraries, enbaling devvelopers to build with .NET across Windows, Mac or Linux'.

This is old story, since 2004 the 'mono project' was a open source implementation of .NET platform.

Now coming to OWIN, which is short for Open Web Interface for .NET OWIN) defines a standard interface between .NET web servers and web applications.

You can find everything you want to know about OWIN here at this master site.
http://owin.org/

These are some of OWIN compatible projects:
Servers and Hosts
    Katana
    Nowin
    Suave
Frameworks
    Jasper
    Nancy
    SignalR
    WebApi
    WebSharper
    DuoVia.Http
    Simplify.Web
    Wisej
Implementations
    Katana
    Freya
    ASP.NET vNext

Microsoft uses OWIN for building ASP.NET Single-Page applications and several of the assemblies used are OWIN based as shown here:

MicrosoftOwinAssmeblies.png