Cortana

If you've been following Windows Phone at all, you'll know what Cortana is: Microsoft's take on a personal digital assistant. The mobile version offers features that are a mix between Siri and Google Now, but the PC version has a few added benefits. Cortana plugs into Windows search, meaning if you ask for "PowerPoint slides about the charity account," it'll search your computer and OneDrive accounts for relevant files. Microsoft is really pushing its natural-language and transcription abilities -- onstage, Cortana was asked to "show photos from December," which it happily (and quickly) did, and also transcribed and sent an email entirely through voice commands.

New, universal apps

Perhaps the biggest news, though, even if it doesn't benefit the majority of Windows users -- is that Windows apps will now be universal and run across PC, tablet, phone and even Xbox One. That doesn't mean PC users won't be getting new apps as well. Microsoft blazed through a load of new apps for Windows. Most are refreshes of existing offerings, although there's an entirely new Office suite that includes a new version of Outlook, which uses the Word engine for composing or displaying emails, and a refreshed Photos app. The biggest new addition, though? Project Spartan.

Project Spartan

As early reports suggested, Project Spartan is a new browser for Windows 10, entirely separate from Internet Explorer. Sure there's a very clean, almost Chrome-like design, and a brand-new rendering engine, but the focus here is on social sharing. You can highlight and annotate websites before sharing them with friends, kind of like having Skitch built right into your browser. There's also Cortana integration and a reading view that, much like Pocket, lets you read pages offline.

Better settings

Windows 10 goes a long way to fixing the fragmented Control Panel and Settings menus currently in Windows 8.1. A unified settings menu is coming, with a clean design and simple options, and there's also the new Action Center (pictured above), which provides more toggles for switching on or off WiFi and other settings. Notifications will also pop up in this space, which will be synced across devices -- if you dismiss a notification on your phone, you won't see it when you next look at your PC, and vice versa.

Continuum

Not just the name of John Mayer's mellow third studio album and an underrated sci-fi series, Continuum is now a big part of improving Windows on convertibles. We got a brief glimpse of it back at Microsoft's last Windows event, but it's now been fully explained. If you have, for example, a Surface Pro 3 with the keyboard docked, the experience will be very similar to Windows 7 (or Windows 8.1 in desktop mode). All apps are now windowed, whether they're the new touch-friendly kind or legacy applications. But say you're using an app like OneNote, and you want to undock your Surface and use it as a tablet: Do just that, and it'll automatically expand the app into fullscreen mode, making it easier to use with your fingers.

Gaming

Sony has let you stream PlayStation 4 games to a Vita from launch. Microsoft's response? To let anyone stream any Xbox One game to any Windows 10 PC or tablet. Sure, you have to be on the same network, but it's still pretty massive news. Elsewhere, there's a new Xbox app for PC and tablet that lets you access your activity feed, messages and friends list. It'll even display information on games in third-party clients like Steam.

Uhh... Holograms?

We were promised a couple of surprises today, and it's fair to say that Microsoft came through with a huge surprise: Windows Holographic. It's an AR platform, letting you see 3D "projections" using a pair of AR goggles. You're probably thinking this is some vague pipe dream, but it's not. Microsoft also showed off HoloLens -- a wearable, wire-free computer that will enable Windows Holographic -- which the company says will be available at some point "in the Windows 10 time frame."

Free upgrades, but we don't know when

While it's not really a feature, it's definitely worth noting: Windows 10 will be free for everyone using Windows 7 or 8.1, provided you upgrade within the first year. As for when you can actually get all these new features on your computer? Microsoft is staying mum about that. The latest preview build, which'll have many, but not all of the features announced today, is rolling out to Windows Insiders later this month, with a phone-friendly version coming in February. A consumer-ready version is still earmarked for "2015."