Virtualization is being adopted in many different environments and situations. If you are a developer, virtualization can provide you with a contained environment where you can safely do almost any sort of development safe from messing up your main working environment. If you are a systems administrator, you can use virtualization to more easily separate your services and move them around based on demand.
The default virtualization technology supported in Ubuntu is KVM. KVM requires virtualization extensions built into Intel and AMD hardware. Xen is also supported on Ubuntu. Xen can take advantage of virtualization extensions, when available, but can also be used on hardware without virtualization extensions. Qemu is another popular solution for hardware without virtualization extensions.