Introducing Kestrel
Kestrel is a newly implemented HTTP server that is the hosting engine of ASP.NET Core. Every ASP.NET Core application will run on the Kestrel server. Classic ASP.NET applications (running on the .NET Framework) usually run directly on the IIS web server. With ASP.NET Core, Microsoft was inspired by Node.js, which also ships an HTTP server, called Libuv. In the first version of ASP.NET Core, Microsoft also used Libuv, and then added a layer on top called Kestrel. Node.js and ASP.NET Core shared the same HTTP server at that time.
Since the .NET Core framework has grown, and .NET Sockets was implemented on it, Microsoft has built its own HTTP server based on .NET Sockets, and removed Libuv, which was a dependency they don't own or control. Now, Kestrel is a full-featured HTTP server that runs ASP.NET Core applications.
The IIS web server acts as a reverse proxy that forwards the traffic to Kestrel and manages the Kestrel process. On Linux, usually NGINX...
