What is "virtualhost"

The concept Virtual Host applies both the practice of machine virtualization and to servers (Database, HTTP Servers, FTP Servers, among others) that meet requests for different domains and network addresses (IP), thus allowing the machine (Host) to appear to be multiple machines.

Example of applications applying the concept Virtual Host as virtual machines on the same Operating System or even Completely Distinct Systems, are the QEMU, Xen, Virtual Box, among several others.

Already applications that use the concept of Virtual Host to answer services on the same machine we have the Apache HTTP Server, and databases like Mysql and Microsoft SQL Server.