Virtualization allows you to run several "seeming-as-if" (virtual) computers on one computer. This can help you

to work in several isolated VPN environments on one computer
to test several Linux distributions running at the same time on one computer
for developers, to test "dangerous" changes like on the login manager or the boot sector
in big companies, to take best use out of your computing resources

Topics

Find out which virtualization you run on

If /proc/xen/control_d exists, you run on XEN virtualization

Solution	max vCPUs	bridged networking possible	Snapshot possible	remarks
KVM	>= 8	yes	no
Virtualbox	>= 16	yes	yes	easy to install and easy to use
VMWare Player	4	yes	no