QEMU is an open-source emulator and virtualizer that allows users to run multiple operating systems on a single machine. It can emulate various hardware architectures, making it versatile for testing and development purposes. Users can run different operating systems, such as Linux or Windows, without needing separate physical hardware. In addition to emulation, QEMU can also be used for virtualization, enabling efficient resource management on servers. It works well with other tools like KVM (Kernel-based Virtual Machine) to enhance performance. This makes QEMU a popular choice for developers and system administrators looking to create isolated environments for testing and deployment.