Microsoft usually brings out new versions of Office to match new versions of Windows, and Windows 8 is no exception. The new Office 2013 is designed to work with Windows 8, and both have a similar modern styling that puts the emphasis on your content rather than Microsoft's software. Also, both are designed to work with cloud (online) services, particularly Microsoft's SkyDrive.
However, other things have changed now that Microsoft is not a software company but a cloud-based "devices and services company". As chief executive Steve Ballmer said last year, this "impacts how we run the company, how we develop new experiences, and how we take products to market for both consumers and businesses."