MonoGame is an Open Source implementation of the Microsoft XNA 4 Framework. It’s always been dependent on XNA, which has been dead for a few years now, making it a little complicated to use. Microsoft discontinued XNA framework development few years back. With the recent release of MonoGame, it has now broken its ties to the old XNA framework and stands alone.
This is an epic milestone as MonoGame finally realises its dream of journeying beyond the realms of what XNA had to offer (some might argue it already did that with all the platforms that MonoGame supports) and is building capabilities that never existed before in XNA. Granted this is not a big rush forward but small baby steps inching forward.
As it stands, MonoGame implements practically the full set of functionality that XNA did, meaning that all your old XNA 4.0 projects will be compatible with MonoGame. There may be one or two little hiccups (this baby is taking its first proud new steps in the world) along the way but it has a proud tradition of hardy developers backing up this project.