WIndows RT doesn't run on Intel/AMD x86/x64 processors - it runs on ARM processors (same things in 99% of all mobile phones and many other electrical appliances). Since ARM and x86/x64 have totally different instruction sets, they can only run apps/programs written for that particular processor - which is why RT is limited to store apps and comes with a specific version of Office.
Another advantage is that a Surface RT would get nowhere near as hot as a normal Surface because ARM processors don't run anywhere near as hot as an Intel/AMD processor, so battery life would be improved.
End of the day - if all you do is a bit of word processing/spreadsheets and playing games from the Store (i think the RT comes with some form of music and video app), that would suffice. Anything more than that you would want a Pro.