Virtualization typically means platform virtualization. It allows you to run one or more operating system images on one computer. For more information read http://en.wikipedia.org/wiki/Virtualization.
Platform virtualization under Linux can be done with
- VMWare - full virtualization
- VirtualBox - like VMWare, but only 1 processor per VM
- KVM - full virtualization (paravirtualization is in test)
- XEN - paravirtualization
- User-mode Linux - pseudo virtualization
- Qemu - processor emulator