Virtualization is term in computer that creating the virtual computer instead of physical system, which performs as real computer. We can define separate memory (RAM), harddisk, network interface and necessary things required for running any PC.
You are running any operating system with high configuration and you want to perform some task or programming in other operating system thne instead of buying new PC/laptop you can separate resource from existing PC and install other OS.
Its not dual boot system, in case of dual boot we install different OS and at the time of booting we need to select however in case of virtual we can run other OS together.
Virtualization can be achived using software call hyper-visor. There are different types of hypervisors available
Native of Bare Metal Hypervisor:
Type 1 or Native or Bare Metal Hypervisors: Run directly on the host’s hardware to control the hardware and to manage guest operating systems. A guest operating system runs on another level above the hypervisor. Example VMware ESXi
Type 2 or Hosted Hypervisors: Run within a conventional operating system environment, which means Type-2 hypervisors run alongside of operating system. Physical Host is installed with Host OS with hypervisor software. KVM, Virtual Box