There are two ways to get Microsoft Office on your Mac. The easiest method is to purchase Microsoft Office 2011 for Mac, which runs natively in the Mac OS X operating system. While the latest edition has almost reached feature parity with the Windows version, you might have some obscure needs which only Microsoft Office 2010 (for Windows only) can fulfill.
If that's the case, you will need to install Windows via Boot Camp or a third-party virtualization program such as Parallels Desktop for Mac. This will allow you to use the Windows version of Microsoft Office, though this method would require you to purchase at least two software packages (Windows OS and Microsoft Office 2010).