Every year Apple has a party for 5,000 people in San Francisco where they announce what they've been working on all year, known as the Worldwide Developer Conference.

This year's event didn't disappoint, with new versions of iOS, OS X and watchOS announced among a range of new features, surprise developments and updates to old technology. Here's everything you need to know in the wake of WWDC 2015.

iOS 9

It's a rarity for Apple to introduce entirely new features to its mobile operating system, and iOS 9 is an excellent example of that. Nearly everything Apple is adding to its flagship mobile operating system this fall (think late September) is a refinement of what came before it.

Siri has been given additional "initiative" with the introduction of Proactive Assistant mode, which uses the context of what you're currently doing to bring you relevant information. It's just like Google Now, providing you apps, contacts and so on based on the time of day, your location, and current activity. Apple has addressed privacy concerns, taking aim at Google Now's intrusive method of combing your data for relevant context.

Security is also on the cards, with the introduction of 6-digit passcodes and two-factor authentication for iCloud, along with assurances that data and usage statistics gathered won't be tied to any Apple ID. In other changes: Maps gets public transport information, Passbook is now called Wallet and there's a brand new News app to replace Newsstand that looks a bit like Flipboard and lets you read your favourite websites.

iPad owners will finally be able to pin a secondary app alongside another by swiping down from the top of the screen. Chat with friends while you study a map, make notes in Evernote while browsing the web, watch TV while you should be working — you get the picture.

OS X "El Capitan"

Last year we had Yosemite, this year it's El Capitan — aptly named after the vertical rock face to the north of the Yosemite valley. Every Mac capable of running OS X Yosemite will be able to handle El Capitan as the iOS-ification of OS X continues.

The changes to Apple's desktop OS don't seem as dramatic as those seen in past updates. Window management has been improved with a refined Mission Control interface that lets you dump your messy windows elsewhere. Spotlight has also been worked on, and can now be interacted with using natural language and features transit, stocks, video, weather and sports results.

The desktop itself gets a Windows Aero snap-style Split View feature and a brand new big cursor mode that highlights where your pointer is when you get back to your machine. Safari sees two key improvements: the ability to pin tabs, just like you can in Chrome; and tab muting.

Notes are now richer and can include images, and Apple's console-level graphics technology Metal makes it to OS X at last (not that you'll want to buy many Mac App Store games). There's also a new version of Maps (with the same mass transit support as seen in iOS 9) and further enhancements to Mail (including swipe gesture support).

watchOS 2.0

Even if you don't have an Apple Watch, it's worth taking note of this gargantuan update, the likes of which we haven't seen applied to any other wearable. To kick things off, Apple will be making the Watch available in seven more countries from June 26: Italy, Mexico, Singapore, South Korea, Spain, Switzerland and Taiwan.

The biggest update to watchOS comes in the form of an update to WatchKit, the toolset that developers use to create their apps. Developers now have access to all of the hardware and sensors on the device, along with use of the digital crown in a customized UI. This update also opens up certain watch face "Complications" to developers too.

Speaking of, there will be two new watch faces added including one that allows you to set your own photo (or an album of photos, which you can swipe through) and another which features various time-lapse shots of London, Shanghai and New York City.

FaceTime calling will also be added with the update, complete with a new Nightstand Mode which allows you to use your Apple Watch like a traditional alarm clock while you charge it. The Apple Watch is also given more independence from the iPhone with WatchConnectivity, a feature that allows third party apps to update their info using known Wi-Fi hotspots (even if your phone isn't present).

Finally, Activation Lock is added which safeguards your Apple Watch against thieves. The update is huge, so keep an eye out for our full low-down when it finally arrives alongside iOS 9.

Apple Music

Rumoured since Apple acquired Beats last year, Apple Music is finally here and it'll cost you $10 a month from June 30, complete with a three-month free trial and $14.99 family plan. It's essentially iTunes, in the cloud, with a powerful search engine attached for finding what you need in that humongous catalogue. Oh, and it's launching worldwide in over 100 countries, simultaneously.

Apple Music is also a radio station, taking over from where iTunes Radio left off by launching Beats 1 — which resembles the BBC's Radio 1 both in terms of faces and branding — and hiring antipodean "man with a voice" Zane Lowe to curate and run the show. It's worldwide, and it's always online — so Zane has his work cut out.

Apple has also shoe-horned social features into the new Music app, under which streaming and radio now live. It's called Connect, and it lets unsigned artists upload their own music and share their sounds with the world while promoting themselves with video, photos and other status updates. It's definitely the least interesting feature included in the overhauled app, especially given Apple's track record with social networks (remember Ping? I didn't think so).

Swift Goes Open Source

Among all the talk about watch faces, music streaming and iOS 9 many failed to grab a hold of perhaps the biggest story of the day. Last year Apple introduced Swift, a revolutionary new programming language that makes it easy to get started programming, removing the reliance on Objective-C and simplifying much of the process involved in turning raw code into finished apps.

Despite a massive surge of interest, many were concerned that developing with Swift would see them hemmed-in to Apple's ecosystem. Apple just burst that bubble by announcing that the language is to be made open source ("later this year"), providing all developers full access to Swift's inner workings regardless of platform. It's worth pointing out you'll still need to pay the $99 developer fee to get your app listed on the App Store, but that doesn't mean you can't build Swift apps for another platform completely free of charge.

"We think Swift is the next big programming language, the one that we'll all be doing application and system programming on for 20 years to come," said Craig Federighi, Apple's SVP of software engineering. It's also been updated to version 2.0, with an assortment of improvements and tools for porting your old Swift 1.2 code.

Apple Pay Slowly Expands

Finishing with (probably) the most disappointing news of the day, Apple Pay will only be reaching the UK with iOS 9 rather than a wider roll-out as many had hoped. While many countries continue to see a surge in popularity when it comes to contactless payment terminals, only British iOS users with cards from MasterCard, Visa and American Express will be able to pay with their iPhones for the time being (including support for London's mass transit).

In the US, the service becomes even more ubiquitous, with the number of stores accepting Apple Pay set to reach over one million next month. The technology can also now be used with store-issued credit and debit cards, and Apple has added support for in-store reward programs too.

What are you most excited about after WWDC 2015?