This article here explains is pretty wellIISReset stops and restarts the entire web server (including non-ASP.NET apps). That includes the following services: IISADMIN, World Wide Web Publishing Service (W3SVC) and Windows Process Activation Service (WAS).
When you do an iisreset, IIS will create a new process to serve requests. All sessions connected to your Web server (including Internet, FTP, SMTP, and NNTP) will be dropped. Any data held in Web applications will be lost. All Internet sites will be unavailable. Then it will try to move all requests on to the new process. After a timeout the old process will be killed automatically.
There are recommendations to restart your web sites in an alternative way by recycling worker processes. More information https://fullsocrates.wordpress.com/2012/07/25/iisreset-vs-recycling-application-pools/HERE