Microsoft stated that Office for Windows RT will come pre-installed on Windows RT devices; however, it will be a preview version of Office Home and Student 2013. Customers will receive a free upgrade to the final version of the suite via Windows Update starting in November 2012 through January 2013.
Office Home and Student 2013 RT will only run on Windows RT machines. It will not be sold as a stand-alone product. Office for Windows RT was built from the same code base as the versions of Office for computers running traditional x86 processors from Intel and AMD, and offers "...the vast majority..." of the features the full PC version of Office Home and Student 2013. According to the blog post by Microsoft, this is the list of missing features for Windows RT users:-
The new Microsoft Office suite (Office 2013) was first unveiled in July this year and its consumer preview, Office 365 Home Premium Preview, was made available for download. For more details about the difference between the desktop Office suite and Office 365, do head over to our Office 2013 FAQs for more information.