Microsoft Corporation is the world's largest software company, and owns a minority sharehold of Bungie.
As Bungie's former parent company, Microsoft has gone to great lengths to supply Bungie with the money, access, resources, and material that it needs while allowing it the freedom to create video games on its own, without excessive meddling.
Microsoft is the owner of the rights to the Halo story as well as all of Bungie's intellectual property and proprietary material. For this reason, Bungie video games are playable only on Microsoft's XBox series of gaming consoles and on personal computers running Microsoft's Windows Operating System.
As of now, Bungie is no longer owned by Microsoft, thus allowing their new game Destiny to be released on the Playstation platforms in addition to the Xbox consoles.
For more information on Microsoft, please visit Microsoft.com.