A Virtual Machine Monitor (VMM), also known as a hypervisor, is software that creates and manages virtual machines (VMs) on a physical computer. It allows multiple operating systems to run simultaneously on a single hardware platform by allocating resources like CPU, memory, and storage to each VM. This enables efficient use of hardware and isolation between different operating systems. There are two main types of VMMs: Type 1, which runs directly on the hardware, and Type 2, which runs on top of an existing operating system. Popular examples of VMMs include VMware, Microsoft Hyper-V, and Oracle VirtualBox. These tools are widely used in data centers and for software development.