Tonight at an event in San Francisco Microsoft's Joe Belfiore and Steve Ballmer revealed the Windows Phone 8 mobile operating system. You can catch up with all the details from Microsoft's Windows Phone 8 launch in our stories: Windows Phone 8 launch: as it happened, and Microsoft launches Windows Phone 8: details. Here we focus on five new features Microsoft promoted at this evening's event.

1. New features in Windows Phone 8: APPS

One of the toughest challenges facing Microsoft as it tries to break open the smartphone market is a lack of apps. Microsoft won't put a figure on the number of apps it has in its store, but also claims with some justification that the critical factor is the number of key apps available. To that end Microsoft announced that the SDK for app developers will be available to all from tomorrow.

Microsoft tonight announced new versions of Twitter and its own Skype internet telephony service. The Skype app is, Microsoft said, always on without draining the battery, like the Skype for Windows 8 app.

Other apps announced tonight include Paypal, Chase bank, and Words with Friends. Indeed, Microsoft said that it expects to soon be at a point where Windows Phone 8 will have 46 of the top 50 apps, working across all Windows 8 devices. Windows Phone 8 apps will move from Windows 8 device to device, of course.

Microsoft also announced that the Pandora music app will reach Windows Phone 8 'early in 2013'. "We didn't want to just bring the same Pandora experience that's on all the other platforms," said Belfiore. "Pandora on Windows Phone 8 will include a free year of music with no ads."

Belfiore was also keen to promote the fact that apps in Windows Phone 8 can be utilised as Live Tiles: fixed to the lock screen of your smartphone, giving out live information even before you unlock your phone.

"We're taking the idea of Live Tiles a lot further," said Belfiore. "The idea is that you can find the things - and people - you care most about... you can choose your favorite apps and have them show you information at a glance. You can reconfigure those tiles to the shapes you like."

Microsoft announced that it had partnered with Facebook to create a live lock screen app.

Lumia Windows Phone 8 devices

2. New features in Windows Phone 8: DATA SENSE

In announcing a new feature Microsoft calls Data Sense, Belfiore said it would let users "surf the web more, and use less data when you do". Up to 45 percent less data, according to Microsoft. Sounds amazing, right?

According to Belfiore Microsoft has built a system in the cloud and on the phone client that compresses every single web page you browse. This shouldn't change the user experience, says Microsoft, but will less data, whilst also helping you find WiFi hot spots on a map.

"Data Sense takes advantage of a WiFi connection when there is one available," said Belfiore. "It can also automatically adjust the way the phone behaves so you won't go over your cap."

The Data Sense Live Tile shows data use in real time, too. It does depend on operators, however, so it will be worth checking that you get it with any Windows Phone you purchase.

3. New features in Windows Phone 8: KIDS CORNER

Another intriguing new feature of Windows Phone 8 is called 'Kids Corner'. This creates a separate place on your phone where children can access only the things you want them to. So they could, for instance play games and watch videos, but nothing else.

To access Kids Corner you head over to Settings, Turn on Kids Corner. There's an off switch for those who don't need this feature, and checkboxes to chose what the kids can access.

The Kids Corner home screen then displays only the apps that you have chosen, in the familiar live tile format. It sounds so simple and useful you wonder why no other smartphone platform has yet provided such a feature.

4. New features in Windows Phone 8: PEOPLE HUB - ROOMS

Microsoft was also keen to promote the virtues of a feature called the People Hub. This is, in Belfiore's words, "the contact list reinvented". Microsoft has built a feature called Rooms into Windows Phone 8. This lets you gather sets of people into groups, the example of which Microsoft gave being the family. Indeed, Belfiore said tonight that the "family room is right in Windows

Phone 8 right out of the box". Within the Rooms you set up you can pan over and send private messages, and share your location. Each room has its own calendar, a place to privately post photos to the room, and an area in which you can share notes only with everyone in the room.

Intriguingly you can invite people to a Room, even if they have a different kind of phone. Microsoft did say, however, that they wouldn't get the 'complete experience'.

5. New features in Windows Phone 8: INTEGRATION WITH WINDOWS 8 PCs & TABLETS

Of course, the biggest thing in Windows Phone 8's favour is the billion or so people who use Windows on PC, laptop or tablet. And Microsoft is determined to make a great deal out of this. Speaking about the combination of Windows 8 and Windows Phone 8 Belfiore said that using a Windows Phone 8 handset offered: "All of your content: office documents, photos, and music wherever you are."

Key to this is Microsoft's SkyDrive cloud computing function. Using Windows Phone 8 and SkyDrive allows you to move files and apps from PC to tablet to phone to Xbox, according to Microsoft, and will "not only feel the same way, but work together in some powerful ways". SkyDrive is built right into the PC and the phone, and is available on the Xbox as well.

If you create a new document with your phone, it can be accessed on your laptop, PC or tablet as well. SkyDrive also supports voice notes. You start with 7GB of free space, and you can pay to add more so you don't have to worry about running out of space.

Similarly you can access Xbox Music service from a Windows Phone 8 device, and this will place your favourite tunes wherever you want them - via Windows PC, Windows Phone and Xbox.

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