Open web interface for .NET is basically a connection between the web servers and the web application. So when you have a startup class with this code in your web app
[Assembly: OwinStartup(typeOf(Startup1))]
public class Startup1
{
public void Configuration(IAppBuilder app)
{
app.run(()=> {
Context.response.contextType = "Text/plain";
})
}
}
So this is a middleware in the OWIN pipeline, when the webserver recieves a HTTP request the OWIN pipeline invokes the middleware. The middleware code will then run.