Microsoft announced Windows Phone 8.1 during the keynote address on the opening day of Build 2014, as expected. Despite the version number, Windows Phone 8.1 is a major update to the world's most personal smart phone platform, as Microsoft calls it, adding deeply integrated voice interaction, a new Action Center, and many other features.

New features and other changes

Windows Phone 8.1 is a major effort. Fortunately, Microsoft nicely telegraphed where we should start.

Cortana. This one is so big, so important, and so potentially earth-shattering that I'll try to cover it separately soon. But the short version is that, yes, Cortana is voice control software like Apple's Siri feature for iPhone or Google Now. But it appears to go well beyond the capabilities offered by those platforms in several ways, the most important of which is deep and immediate integration with third party apps like Facebook and Twitter.

Action Center. Like the notification centers in Android and iOS, the new Action Center in Windows Phone 8.1 combines quick settings (which are customizable, nice!) and app notifications. You access Action Center by swiping down from the top of the screen, naturally.

Word Flow "shape" typing. Microsoft's virtual keyboard for Windows Phone, Word Flow, is getting updated with Swype-style "shape" typing, where you swipe you finger from key to key as you type. Microsoft claims this new keyboard is now the fastest "shape" typing keyboard in the world.

Lock screen themes. One of the big, um, themes in Windows Phone 8.1 is extensibility: It seems like every nook and cranny of the OS is being opened up to customization, including by third party apps. Case in point, the lock screen, which will now support themes that provide custom visuals and animations. The few they demonstrated at Build were quite attractive.

Start screen background. As expected, Microsoft is letting users customize the Windows Phone Start screen background with custom photos and pictures. But it doesn't work like it does in Windows, where that image peeks out from behind the tiles. Instead, Windows Phone puts the background only on the tiles—not on the space between those tiles—leading to some very interesting effects.

(Deeper) Skype integration. Today, you can get a Skype app for Windows Phone and it can integrate into the OS in some interesting ways. But in 8.1, this integration goes deeper: Skype is integrated with the Phone software, so you can "hand off" a phone call to Skype, turning it from a phone call into a video conference.

(Improved) Calendar. The Calendar app is getting a nice functional bump with a new week view that expands and contracts as you tap and, as important, the ability to swipe to the right to see what's happening tomorrow, next week, or next month.

Podcasts, Xbox Music and Xbox Video apps. The Music + Videos hub is no more, replaced by three separate apps: Podcasts, Xbox Music and Xbox Video. These (and other) apps benefit from the new volume controls, which differentiate between ringer/notifications and apps/media. I wrote about the Podcasts app previously Windows Phone 8.1 Preview: Podcasts.

(Improved) Photos app. The Photos apps is now extensible so that third party services can integrate with it and users can view their photos no matter where they are.

Wi-Fi Sense. This new settings app will automatically connect to free, high-quality public Wi-Fi networks using a crowd-sourced database of known-good networks.

Settings sync. Like Windows 8.1, Windows Phone 8.1 will sync tons of settings through your Microsoft account and those that make sense will sync between Windows and Windows Phone too.

Universal apps. Microsoft is helping developers create apps that run on both Windows and Windows Phone (and, soon, Xbox One), via a new Universal Apps capability. And the firm is improving its Store infrastructure so that you can buy an app or game on Phone and then get the same app or game on Windows (or vice versa) for free. I wrote about this previously in Windows Phone 8.1 Preview: Universal Apps.

External display. Windows Phone 8.1 supports Miracast for wireless display projection, but it can also display over USB.

Internet Explorer 11. The latest version of Microsoft's web browser is coming to Windows Phone, as you'd expect. It brings cross-device tab syncing, better standards support and performance, InPrivate browsing, password sync (with PCs) and a gorgeous new Reading Mode (as in Windows).

Schedule

Microsoft previously promised that all existing Windows Phone 8 devices would be freely upgradeable to this new version, and the firm indicated at the show that this is still the plan. (Former Nokia CEO Stephen Elop explicitly stated that all Windows Phone 8-based Lumia handsets would get this update.) The question of course is when this will happen.

I can tell you this: Windows Phone 8.1 is late. Microsoft had hoped to finish the code for this release in time for Build 2014 so that it could hand out the code and associated developer tools to developers. Despite a herculean effort that included bringing people over from the Windows team to help out, they missed the mark. So things are happening a bit more slowly than was previously hoped. Here's a rough new schedule:

April 2014. Sometime later this month, Developers who partake in Microsoft's free or paid Windows Phone developer programs will be able to get a beta version of the Windows Phone 8.1 SDK. Read Windows Phone 8 Update 3: Update Your Handset Today! for details on this program.

May 2014. The first handsets based on Windows Phone 8.1, the Lumia 630 and 635, will ship internationally.

June 2014. The Lumia 930 (basically a GSM version of the Lumia Icon) will ship internationally.

July 2014. Windows Phone 8.1 will ship as an over the air update for existing phones. This update will almost certainly take a few months, as with previous updates.

I'll be writing a lot more about Windows Phone 8.1 soon, including updating my free 650-page Windows Phone 8 e-book to a new title that will be called Windows Phone 8.1 Field Guide. More soon!