If you're considering buying Office 2013 there are a few things you should know first.
Office 2013 does not include an update for the Mac OS X version of Office. Mac users can still purchase a single license of the existing version (Office for Mac 2011) or install Office for Mac 2011 on up to five computers with a subscription to Office 365.
A subscription to Office 365 also guarantees that you'll receive the newest version of Office for Mac whenever it's made available.
If you have a Microsoft account, you can use Office Mobile apps to create and edit Office documents from various devices, including iOS, Android, and Windows Phone. The basic editing features are available for free, but to use all of the features you'll need an Office 365 subscription.
Keep in mind that Office Mobile apps don't include all of the features found in the desktop version of Office. They will probably be most helpful for editing existing files on the go.