The world's most used operating system has made it into double figures.

Windows 10 is destined to change the way millions of us interact with our computers and gives Microsoft's chief executive, Satya Nadella, the opportunity to steer the company in a new direction.

He describes the OS as "Windows as a Service", which means he plans to release improvements as they become available, via the internet, rather than working towards another "big bang" upgrade in a few years.

Versions of the software will also be released for smartphones - making the Windows Phone OS defunct - as well as the Xbox One games console, "internet of things" kit and, potentially, smartwatches at a later stage.

There has even been talk of this being the "last version of Windows" assuming all goes well - so no pressure then.

Wait... what about Windows 9?

The best explanation Microsoft has given for jumping a number has been in the form of a very nerdy joke: "Windows 10 because seven, eight, [ate] nine."

The suspicion is the company's marketers just think it sounds cooler.

And, of course, there is the added benefit that it further distances the product from the ill-fated Windows 8 while catching up in numeric terms with Apple's rival Mac OS X.

So, what's new?

At first look, at least on a desktop PC, the user interface looks pretty similar to the popular Windows 7.

Microsoft is pitching the product as being "familiar and easy to use", as part of a shift away from the unpopular touch-centric focus of Windows 8.

Perhaps the headline feature is the return of the Start Menu, providing users with quick pop-up access to their favourite apps, documents and settings.

It does, however, carry over some of W8's design language.

To the right of the list of sober-looking shortcuts, the Start Menu now also offers a selection of customisable "live tiles".

These can also launch favoured apps, but people may prefer to use them to get at-a-glance updates about how many unread emails they have, what the weather is doing, forthcoming diary alerts and other notifications.

Those pining for Windows 8's full-display Start Screen can still turn it back on via settings, and it appears by default on tablets when no keyboard is attached.

The other big addition is Cortana - the virtual assistant first featured in Windows Phone 8.1.

This makes sense of natural-language requests spoken or typed into a computer to help users manage their diaries, search the internet for information tailored to their interests, play music they might like, and control third-party apps, among other activities.

The more Cortana studies its owner's habits, the better, in theory, the suggestions it makes. Those who find this a bit freaky can restrict the data it has access to.

Other new features include:

§  Windows Hello - a biometric authentication facility that lets people unlock their computers and specific applications by providing a fingerprint or facial recognition scan

§  Edge - a successor to Microsoft's Internet Explorer web browser that loads sites faster, lets users scribble over and share webpages, and brings up contextual information about selected words at the click of a button, saving the need to type queries into a search engine

§  A notifications sidebar - clicking on the Action Center taskbar button now lets users review recent notifications. This addresses a common complaint that once notifications had slid off the screen in earlier versions of the OS, they were gone for good

Microsoft has a lot riding on this, doesn't it?

The money paid by computer makers to preinstall Windows on their PCs is one of Microsoft's biggest sources of revenue.

It also helps Microsoft direct business' budgets towards its lucrative add-on cloud services - including its Office 365 productivity suite, Azure cloud computing platform, and Dynamics supply chain management tools - as well as the IT support and training it sells.

There are also subscription fees to be made from consumers, whether it is charging for OneDrive cloud storage, calls made via Skype, or access to Microsoft's Groove music library.

Every customer lost to devices running a Google, Apple or Linux operating system is one more likely to spend their money elsewhere.

In addition, every company that decides to stick with Windows 7, Vista or even XP is one that will not be able to support all of Microsoft's latest technologies.

Windows Phone has also been a bit of a flop, at least in most countries.

Now, there is a new pitch: if you link a Windows 10 Mobile handset to a screen, keyboard and mouse, it can double as a PC.

This feature - called Continuum - might appeal to the billions of consumers in emerging economies for whom buying both a smartphone and a laptop would be unaffordable.

What's Microsoft doing to give Windows 10 a better chance?

Consumers and small businesses are being offered a free upgrade of existing computers running Windows 7 or 8 at any point over the first year of Windows 10's release.

In addition, Microsoft's move to a "universal app platform" should make the OS more attractive to developers.

Microsoft will run a single app store, and every product should be compatible with a plethora of devices - including PCs, Xboxes, smartphones and the forthcoming HoloLens headset - as long as the computer involved is powerful enough and the coder has made the user interface responsive.

Microsoft is also making it easier for developers to port Android and iOS apps to the platform.

But I've heard it's a bit flaky

Until about a month ago, this was a common complaint about the preview builds.

But critics acknowledge many of their biggest gripes have now been addressed, even if there are some outstanding issues - such as Edge's lack of support for extensions.

Big businesses are still unlikely to adopt the OS on day one. Tech consultancy Gartner has advised its clients to wait until at least the second half of 2016.

However, one issue that may have deterred consumers has been addressed.

Microsoft caused controversy when it suggested people using the Home version of its system would have to add new features as they became available.

Matters were not helped when it explicitly said businesses would be able to wait until "quality and application compatibility has been assessed in the consumer market".

That led some bloggers to accuse Microsoft of treating the public like "guinea pigs".

But on the eve of Windows 10's release, Microsoft told the BBC it had added an option to defer all Home Edition updates, security fixes aside.

There remains, however, an incentive to buy the Pro or Enterprise versions, as their owners will be able to control which specific features are added.

One final thought: just in case you try Windows 10 and do not like it, it is worth knowing there is a "rollback" option that works for the first 30 days.