You're viewing the documentation for the experimental version of Vagrant (the 1.x series). The current stable series is 1.0.x and the documentation can be found here. You can learn more about the Vagrant versioning scheme here.
Configuration key: config.vm.guest
Default value: :linux
Some actions that Vagrant must do to the virtual machine require operating system specific behavior, such as mounting shared folders or configuring the network. Vagrant includes support for many well known guest operating systems and this configuration option specifies what guest OS will be installed. By default this is set to :linux.
For more information, please read the documentation on guest-specific behavior.