Any system offering the user the choice of two operation systems (OSes) under which to start a computer. A dual boot system allows the user to run programs for both operating systems on a single computer (though not simultaneously). The term "multiple boot" or "multiboot" extends the idea to more than two OSes.
The OSes are generally unaware of each other's existence. They are installed on separate hard disk partitions or on separate disks. They may be able to access each other's files, possibly via some extra driver software if they use different file systems.
The OSes need not be completely different - they might be different versions of Microsoft Windows (e.g. Windows XP and Windows NT) or Linux (e.g. Debian and Fedora).
A dual boot system differs from an emulator such as vmware, which runs one or more OSes "on top" of the primary OS, using its resources.