Windows 8 is a totally new version of Windows that, in addition to the traditional desktop, also includes a new-style interface for use with touchscreens - whether that's on a touchscreen laptop, all-in-one PC or tablet.
And, while not all PCs will be touchscreen when Windows 8 launches, expect more and more devices to have touchscreens towards the new year - even if it's a traditional laptop.
Windows 7 was a big hit for Microsoft, turning things around from the troublesome Windows Vista and reminding people that the Redmond giant was not quite ready to hang up its hat just yet.
The Windows 8 release date is here and the challenge for Microsoft is how it builds on the success of Windows 7 and show that Windows can work on iPad-like tablets. But it still needs to dominate on laptops and desktops.
Windows 8 isn't a phone OS - but does share a great deal of design language and code with its sibling, the new Windows Phone 8.
Here are 8 key differences that Windows 8 brings to the table.
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1. Windows 8 touch
Obviously the most obvious difference between Windows 8 and its popular predecessor is the user interface.
Windows 7 supported touch, but it wasn't ideal - the controls simply weren't good enough. However, things have improved immeasurably in terms of the touch support in Windows 8. First of all, touch support on the Desktop is far, far better and you can even close windows and select menu items without issue - Windows has built-in intelligence to tell it what you are trying to do.
Secondly, the new Start Screen is an interface that's built for touch. That means tiles instead of menus and much quicker ways to get to the programs you want. There's also greatly improved on-screen keyboard and handwriting recognition.
You don't need to have a Windows 8 touch PC - the interface still works on non touch machines, and many trackpads have support for new Windows 8 gestures. There are also peripherals such as touch mice and trackpads from Microsoft and Logitech (among others) that support Windows 8 gestures.
2. Windows 8 Start screen
Microsoft the familiar Start menu is a Start screen which features the same kind of live tiles and data as Windows Phone's home screen,
When you open an app that needs the desktop you still get the familiar Recycle Bin and Taskbar, but the Start button - which now only appears when you hover in the bottom left corner with your mouse - takes you back to the Start screen.
The Start Screen can be used as an application launcher for desktop apps, or Windows 8 Modern UI apps (that's what Microsoft is currently calling the new interface).
3. Better multiple monitor support
Microsoft has decided that, with more of us using multiple monitors on our PCs, that it needed to overhaul its desktop management.
That means you can now have the Start Screen on one monitor and the desktop in another, or choose to have the Windows 8 Desktop and taskbar on both screens.
You can also put a different background on each screen if you have multiple monitors. Windows 8 also enables you to split screen between Modern UI Windows 8 apps, so you can have both your Windows Messenger on a third of the screen alongside your Desktop. This takes a bit of getting used to!
4. Windows 8 charms
A key arrival for Windows 8 is what Microsoft is calling Charms. These appear when you mouse to the right-hand side of the screen or swipe in from the right on a touchscreen.
They enable you to access the Start Screen on a touch device (although many touch devices will also have a physical Windows 8 button on the bezel of the screen or a Windows key on the keyboard).
The other buttons are Search, Share, Devices and Settings and provide quick access to these functions on touch and pointer-driven displays alike.
As well as searching your apps and folders, charms work across different apps, so for example a social app can tap into the Share charm so you can share files to that app quickly and easily - it's contextual to the app you are using.
The Settings charm gives you quick access to basics such as volume and brightness controls, as well as putting your PC to sleep or restarting it.
The search and devices charms are fairly self explanatory, but the share charm is interesting.
5. WIndows 8 Search and Social
With the Start menu gone, search is available not only through the Search charm but also through the Start Screen - just start typing and the results on screen are for programs and files.
As with Share, the Search charm is contextual, so you can search inside any app - for example you can do a web search from here, or look for a destination using the Travel app. Doing a web search is powerful and quick, it's a simple way to launch a browser and search speedily.
As for social, WIndows 8 supports Facebook, LinkedIn and Twitter natively, so you can browse social updates within the People app and elsewhere.
6. Windows 8 ARM support
Until now, Windows has only supported x86-based Intel and AMD PCs but that is all changing with Windows 8, which will support devices running on ARM architecture.
British company ARM's chip designs are being used in a growing number of devices, and Microsoft is keen to make Windows as widely available as possible - especially on cheaper Windows 8 tablets to compete with the iPad and Android tablets.
While ARM produce the original so-called 'instruction set' for the processors used in these devices, other people manufacture the chips. So expect to see Windows 8 tablet devices based on Nvidia's Tegra 3 and Qualcomm's Snapdragon to start with.
The version of Windows 8 used on ARM-based devices is actually referred to as Windows RT - this stands for runtime. You can't buy this separately, only with a device.
7. Windows Store
Microsoft's Windows Store is a key part of Windows 8, offering both desktop and Modern UI apps, both free and paid. You can search the Store using the Search charm, as well as browse through the top free or top paid apps as well as look through apps by category.
When apps are updated, you can also download these updates very easily, just as you would on iOS or Android.
8. Windows 8 cloud integration
While Microsoft may not agree with Apple's Steve Jobs that we are in a post-PC era, it is clear that they are keen to make cloud integration central to Windows 8.
That means the potential to sync data to SkyDrive - there's a SkyDrive app as well as the ability to save data to and from your cloud storage. Office 2013 apps have SkyDrive capabilities included, too.
Microsoft also syncs settings your Windows 8 PCs - including your browsing history in IE, for instance. Photos can also be shared across multiple PCs.