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OWIN is a standard for an interface between.NET Web applications and Web servers. It is a community-owned open-source project. Prior to OWIN, Microsoft's ASP.NET technology was designed on top of IIS, and Web applications could not easily be run on another Web server. OWIN aims to decouple the relationship between ASP.NET applications and IIS by defining a standard interface. Developers of Web servers can be sure that, if they implement OWIN correctly, ASP.NET applications will run on their server. Similarly, new Web frameworks could be developed as an alternative to ASP.NET. So long as they target OWIN, they will run on any OWIN compatible Web server, including IIS.
In this regard, OWIN aims to do for.NET what Java Servlet and Servlet containers do for the JVM.
Project Katana is a set of OWIN components built by Microsoft.