VirtualBox is a virtualization application that lets one computer run several other operating systems at once, each inside its own virtual machine. Oracle’s manual describes it as “a cross-platform virtualization application,” explaining that “it extends the capabilities of your existing computer so that it can run multiple operating systems, inside multiple virtual machines (VMs), at the same time.”
Crucially, VirtualBox is a hosted hypervisor rather than a bare-metal one. The documentation states: “Oracle VirtualBox is a so-called hosted hypervisor, sometimes referred to as a type 2 hypervisor. Whereas a bare-metal or type 1 hypervisor runs directly on the hardware, Oracle VirtualBox requires an existing OS to be installed.” This is what makes it convenient on ordinary desktops and laptops, where it runs as just another application.
Its reach across operating systems is a defining trait. The manual notes that VirtualBox runs on multiple host operating systems, including Windows, macOS, Linux, and Oracle Solaris, so the same tool and the same virtual machines can move between very different host platforms.
VirtualBox was first released in 2007 by the German company Innotek, then passed to Sun Microsystems and, after Sun’s acquisition, to Oracle, which continues to develop it. The project’s public changelog tracks an active stream of releases, and its combination of zero cost and cross-platform support has made it a standard tool for developers and students running test environments on their own machines.