[Update: new media available]
Today at GDC 2013, Hideo Kojima revealed that Metal Gear Solid Ground Zeroes and The Phantom Pain make up Metal Gear Solid V. You can check out the trailer for Metal Gear Solid V: The Phantom Pain below featuring over five minutes of gameplay:[youtube http://youtu.be/xw5t45tEPWQ]
Metal Gear Solid Ground Zeroes acts as a prologue to The Phantom Pain. According to Kojima, The Phantom Pain is the main game. As seen in the trailer, The Phantom Pain opens up with a tutorial where Snake wakes up from a 9 year coma.
The game is currently running on PC now on the new FOX Engine (for development) but has been announced for PS3 and Xbox 360.