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Microsoft is the name of the company building Windows and Office and some other tools.
MS Office 2011 for Mac is programmed to run on Mac OS X. It will run on Mac OS X without the need to partition the HDD/SDD, as MS Office is only an application suite and not an OS as MS Windows (XP, Vista, 7) is, which needs its own partition, but MS Office 2011 will install like many other Mac OS X applications into the Applications folder.

MS Office 2010 for Windows vs. MS Office 2011 for Mac
 
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