Windows RT (and Windows RT 8.1) is available on several devices that run ARM processors. Standard Windows will not run on an ARM processor so this special version of Windows 8 was created to do just that and dubbed "Windows RT".
You cannot run x86 apps on Windows RT. Just apps that are available to download through the Windows Store.
The original Surface and the Surface 2 run Windows RT (and Windows RT 8.1)
Also, the Asus Vivotab RT, Dell XPS 10, Ideapad Yoga 11, Samsung Ativ Tab and Nokia 2520 were the devices made to run Windows RT.
Windows 8.1 should continue to see security patches through EOL but it's doubtful it will receive any more significant changes.
It might get the Edge Browser?
I'm in the Love Windows 8.1 camp. I see no problem with Windows 8.1 and think it's awesome, especially on my Windows tablets.
I went ahead and upgraded one of my tablets to Windows 10 last night. it's the first time I have had hands on experience with the OS.
I miss the charms bar and the sliding the programs in from the left side. The snap doesn't work as well IMO. The Tablet Mode is not as robust as the Metro interface for tablets in Windows 8.1.
I guess I have to get used to it. I have friends at work with 8.1 tablets and they bring it to me and I can swipe around so fast and fix what they need they just can't keep up with what I am doing. LOL. Windows 8.1 works magnificently on tablets!
And with PCs too. I tried Windows 10 on an aging HP small form factor machine with a P4 3 GHz. It only has a single gigabyte of RAM, and a 20 GB HDD. Windows 10 started up faster, but I just installed Windows 8.1 Single Language (non-Activated) and it runs ever more quickly, Startup is a hair longer but navigation of the OS is more brisk.