The overly-simplified situation is that when buying an OS with/for a PC/laptop Windows 8 is the next version of Windows and will run traditional Windows apps as well as WinRT apps from the app store (although I'm not 100% sure on that last bit). If you buy a lightweight "Win8" tablet and aren't spending more than a high-end iPad you will get Windows RT which does not run traditional Windows apps at all, but will run WinRT apps from the app store that nobody has programmed anything for yet. (Perhaps a slight exaggeration on that last point.)
Things get really confusing because they keep changing the names of things:
Windows Runtime (WinRT): An application framework MS wants developers to use for Windows 8 and Windows RT.
Windows RT: The market name of the OS for tablets that are cheaper than high-end iPads. (More specifically, ARM-based tablets and possibly small netbooks if some manufacturer thinks that form factor will sell with Windows RT.) Wikipedia says "RT" doesn't stand for anything, but it looks suspiciously like WinRT/Windows Runtime to me.
Metro UI: What Microsoft called the new interface until it recently decided it didn't like that name anymore.
Modern UI or Windows 8 UI: Apparently what they want to call it now...or at least last week. Who knows what they're calling it now? In any case, whatever they call it takes up the whole screen and is what you see instead of the start menu.
I've been running the Consumer Preview Win8 for a few months on my PC. I had really grown to like the Win7 start menu and miss it, but I can still search by typing when the tile UI is showing, although apps, files and control panel items are three different results sets that aren't shown at the same time, whereas in Win7 it will show you some of each and usually get what I want in there.
The desktop sort of still exists as an application, but any attempt to bring up a start menu takes you back to the Metro UI. But the Win7-style task bar is there on the desktop, and you can pin apps to it and place shortcuts on the desktop. You can also mirror the taskbar on multiple displays, confine it to one or stretch it across multiple displays, so that part is cool.
I have not grown to like the Metro/Modern UI, and I mostly avoid it. As such I find it jarring when launching a PDF, photo or media item from the desktop opens a Metro UI app, so I've generally found ways to stay in the desktop, although I am slowly learning to navigate the Metro UI. Initially I was frustrated that closing apps was so freakin' hard, but it turns out I'm not supposed to worry about that. Modern UI apps stay in the background and eventually die off if you are doing other things and the system needs the memory.
Using a mouse on a PC, the four corners are the key to faster navigation. The upper-left cycles through apps (including the desktop); pulling down from the upper-left gives a list of open windows and moving to the upper-right and pulling down brings up several useful things including the most convenient way to shut down or reboot the PC.
Perhaps I'll eventually learn to use and like the Metro UI, but for now I am barely tolerating it but mostly not caring because I stay on the desktop.
But Windows 8 is faster, and I have had almost no issues with running legacy software or older hardware (stuff that works on Win7, anyway). The only compatibility problem I can think of was that Netflix suddenly didn't think my system time was set correctly and refused to play. I searched the internet and renamed a file to get it going again. I'm not sure that was a Win8 issue, but it seemed like several Win8 users had the problem.
If anyone is going to use Windows Media Center as a dedicated DVR, be aware there are no improvements from Win7 to Win8, and in fact they are taking out the DVD reader codec in Win8 to save a little licensing money. So your Win8 DVR won't play DVDs. There is a way to get that back, but you have to get a more expensive version of Windows *and* pay extra for the codec. So my dedicated DVR that boots straight into Media Center will stay Win7 because I like to use it to play DVDs because my Blu-Ray player is slow and difficult to control. (Because it's old and cheap.)
Once my Consumer Preview expires I'm not sure if I'll go back to Win7 or get Win8 on my PC.
As for the tablets, I'm getting my popcorn ready. MS has marketed Windows 8 as a common OS for desktops, tablets and phones. But this Windows RT-on-ARM tablet that people are going to be comparing to iPad and Android tablets won't run existing Windows apps, and I'm not sure the consumers at large get that yet. The tablets that will run existing Windows apps already exist with Win7 on them but almost nobody buys them because they're really expensive.
I may be confused, but my current understanding is that apps developed in Windows Runtime will run on both Windows RT tablets/phones and traditional PCs running full-blown Windows 8. So if Microsoft can pull this off it could be pretty neat. But my assumption so far has been that their foray into tablets and app markets will fall flat on its face and Windows 8 will be like Vista: it comes on new PCs, but almost nobody upgrades to it and businesses wait for Windows 9. Heck most businesses I've seen are still rolling out Windows 7 and have a significant portion of their userbase on Windows XP.
Additional note: My brother-in-law had been holding out for the Win8 tablet for a year or more, but he just bought a Toshiba Ultrabook (a PC version of a MacBook Air) instead. I haven't inquired as to why he seems to have lost his Win8 tablet enthusiasm.