OWIN (Open Web Interface for .NET) provides a common interface between .NET web servers and web frameworks. It allows frameworks like ASP.NET to run on non-IIS servers and supports new frameworks on Linux. OWIN defines an application delegate that frameworks use to interface with servers. Middleware components can be added to this delegate to modify request/response handling. The OWIN NuGet package provides an IAppBuilder for configuring and launching OWIN-based applications.