Virtualization allows multiple operating systems and applications to run on a single computer using a hypervisor. It is considered green computing because it decreases energy usage and toxic waste by reducing the number of physical devices needed. There are several types of virtualization including server, application, network, storage, and desktop virtualization. Server virtualization specifically allows many virtual servers to run on a single physical server, decreasing energy usage and saving floor space. Overall, virtualization improves hardware utilization and flexibility while lowering costs and environmental impact through reduced resource consumption.