IIS is Microsofts implementation of a web server for Windows. It’s free to use with Windows Server and can even be deployed on a Windows 10 machine. IIS is an easy to use GUI and may be preferred over Apache or NGINX when building web apps that fully utilize the Microsoft Stack.
XAMPP is a package that installs Apache, MariaDB, PHP and Perl onto Windows, Linux or Mac. It is a very simple way to get a web development environment up and running with little knowledge.
Virtual Hosts are used in Apache to allow a single web server to serve numerous websites. Basically www.FailedNormal.com point to /var/www/FailedNormal, and www.EliTheComputerGuy.com points to /var/www/EliTheComputerGuy.