Microsoft has released Office 2013 along with a subscription-based version of the software called Office 365. For Windows users, Office 2013 and Office 365 are all-new versions of the venerable office suite. The major difference between them is that you pay a one-time licensing fee for Office 2013 and a monthly or yearly subscription for Office 365.
Courtesy of Microsoft
Office 365 is available in various versions, but the most popular version for home and home business use will be Office 365 Home Premium, which includes a license for up to 5 PCs and Macs (mix and match as you see fit) for $99.99 per year or $9.99 per month. One thing to note about the current Office 365 Home Premium is that while Microsoft seems to suggest that there is a Mac version of this product, there actually isn't. What you get instead is a copy of Microsoft Office for Mac 2011, which, aside from being updated to support the subscription model, has no new features.
If you have a mixed computing environment and will use all 5 licenses, then Office 365 may be a good product to consider. However, if you just need Office for your Mac, I can't see any advantages in the Office 365 subscription model. That may change at some point, especially if Microsoft creates a truly new version of Office for Mac.
Both Office 365 Personal Edition and Office for Mac 2011 are available as separate products at the Microsoft web site.