Goobuntu is basically a long term support (LTS) version of Ubuntu with custom security features. Ubuntu (pronounced oo-BOON-too) is an open source Debian-based Linux distribution managed by the private company Canonical Ltd. Although new versions of Ubuntu are released every six months, the LTS version is released every two years, making it a good choice for large enterprise customers like Google.
According to Google engineer Thomas Bushnell, Google partners with Canonical's Ubuntu Advantage support program and Google administrators use Advanced Packaging Tool (APT) and Puppet for desktop management. Employees are encouraged, but not required, to use the OS -- but all Google's development tools are built for Goobuntu. The OS does not have a default graphical user interface (GUI); employees can use Ubuntu Unity, GNOME, KDE, X-Window or X-Terms.
Google engineer Thomas Bushnell explains why Google uses Goobuntu.