Think of RT as a *subset* of Windows 8. Full Windows 8 (and 8.1) can run anything a typical Windows PC can - programs from the Internet, off CDs, etc. as well as anything in the Windows Store.
Windows RT works like an iPad or iPhone - you can only install software from the dedicated store, and this software has certain limitations as to what it can do (which is both good and bad - good because it means that programs can't hard your system/data or reduce performance, but bad because some things aren't possible to do with these kinds of apps).
Also...which probably is what causes your confusion...Windows RT has the old Windows desktop as well as Microsoft Office. These are 'added bonuses' of Windows RT, but you can't go and add your own downloaded/CD software to the desktop on RT - you can only use the software that came pre-installed on the desktop.
If you understand how an iPad differs from a PC then you can think of RT as an iPad-like OS (even though it does have a few more capabilities).