It is an online multiplayer spin-off of the Metal Gear video game series. Τhe Starter Pack of Online was available worldwide bundled with Metal Gear Solid 4: Guns of the Patriots, with a standalone release for Japan. Metal Gear Online 's North American and European servers were shut down on June 12, 2012.
On June 13, 2013, Hideo Kojima officially confirmed that Metal Gear Online will be included with Metal Gear Solid V: The Phantom Pain.
Source: Wikipedia, "Metal Gear Online", available under the CC-BY-SA License.