Microsoft's Surface RT tablets include 2013 RT flavors of Word, Excel, PowerPoint, and OneNote, optimized for touch gestures. Office RT generally offers more features than Office for Windows Phone 8 or Office Web Apps, but fewer than Office 2013 on a PC. Surface RT initially shipped with a preview of Office Home & Student RT, with the final version available last October. Microsoft's beefier Surface Pro tablets, on the other hand, can run any iteration of the latest Office software you want to purchase.
Office for Windows Phone 8
All Windows 8 phones run mobile editions of Office software, updated in October 2012. You get to Word, Excel, and PowerPoint through the mobile Office Hub. You'll also get the OneNote note-taking app. Office docs render beautifully on Windows handsets, and they're easy to open from or attach to email messages. Since documents are stored in SkyDrive, they'll display your latest changes and time stamp, whether you've last worked on them from a PC, a Windows phone, or a Windows 8 tablet.
Office Web Apps
These are pared-down versions of Word, Excel, PowerPoint, and OneNote that run in a browser. Panned as weak in the past, they were updated last fall. They're not meant to serve as your primary document tool; rather, they're built to let you access and edit Office files on the go.
Office on Demand
Need to jump onto someone else's computer and work on an Office project? Maybe you need more functions than the Office Web Apps provide. Microsoft tailored Office on Demand for exactly this scenario. Through the magic of virtualization, it lets you run your personalized, full Office applications on PCs where they're already not installed. You just need to be on a Windows 7 or 8 computer, and have a subscription to Office 365. The programs include Word, Excel, PowerPoint, Access, Publisher, Visio, and Project. You log in to your Microsoft account at Office.com to get started; this video explains more.
Office for iOS and Android