What is virtualisation and why is it used?

Virtualisation runs multiple isolated operating systems on one physical machine. A hypervisor sits between the hardware and the guests, allocating CPU, memory, storage, and network to each.

• Type 1 (bare metal) — runs directly on the hardware. VMware ESXi, Microsoft Hyper-V, KVM. Used in datacentres because there is no host operating system consuming resources or adding a failure point.
• Type 2 (hosted) — runs as an application on a host OS. VirtualBox, VMware Workstation. Used for development and testing.

Why it is used:

• Consolidation. Physical servers typically ran at very low utilisation; virtualisation puts many workloads on one machine, cutting hardware, power, cooling, and rack space substantially.
• Isolation. One virtual machine crashing does not affect the others.
• Snapshots. Capture a machine's state before a risky change and roll back in seconds — enormously valuable for patching and testing.
• Live migration. Move a running VM between hosts with no downtime, which makes hardware maintenance possible during working hours.
• Rapid provisioning from templates, in minutes rather than weeks.
• Disaster recovery. A VM is a set of files, so replicating it to another site is far simpler than rebuilding physical hardware.

Note: Containers are the natural follow-up — they virtualise the operating system rather than the hardware, sharing the host kernel, so they are far lighter and start in under a second, but with weaker isolation. The two are complementary, and containers usually run on virtual machines.