Apple’s latest and greatest, iOS 5, is out for download. You’ll be able to load it up on your iOS device and ogle in the wonder of unobtrusive notifications, Twitter integration and much more. But what should you expect, when, and how to get it? Allow us to explain.

What’s in iOS 5?

Loads and loads and loads – more than 200 new features in fact. The basic home screen layout hasn’t changed, but just about everything else has. We’ve written about all the new services in details before, but these are the ones that have impressed us the most:

Notification Centre – No more annoying pop-ups that interrupt whatever you’re playing. Instead, alerts from all your apps appear in this drop down tray at the top of the screen – and the lock screen as well, where sliding an alert immediately launches the relevant app. Nothing groundbreaking here, but very, very polished.

iMessage – Chat with other iOS users seamlessly. Your conversations are tied to your account and so carried across devices. Given BlackBerry’s recent woes, it could prove tempting indeed.

Camera – New image editing tools in Photos mean Apple is taken Instagram and Photoshop on at their own game, but really, we’re just happy to be able to use the volume control as the shutter button once again.

Cable free file transfer – You’ll need to use your charging cable once more to update, but after that, finito. Further software updates will be delivered incrementally and over the air, and Wi-Fi Sync means you can FINALLY update your playlists and media without having to look directly at the atrocity that is iTunes on a computer every time.

AirPlay mirroring – This one’s reserved for the iPad 2 and iPhone 4S only, but it’s a doozy: you can wirelessly transmit your device’s screen to a second-gen Apple TV. That gives you the potential for big screen gaming, easy video streaming, boring photo slideshows and more. Real Racing HD on the iPhone 4S will even allow for four player split screen this way, which looks astonishing. Where’s Mario Kart for iPhone when you need it?

There’s a whole host of other new features, including a new version of Safari that strips out ads and saves stuff for later, Twitter integration and even text expanding shortcuts.

iCloud

iCloud isn’t just for iOS 5 devices (it works with desktops too), but it’s a major feature of them nonetheless, not least because you may never even notice it’s there. It works by backing up all your data for all your apps, and shoving docs and photos into the ether automatically. 5GB of free storage might sound measly, but remember that app and media purchases on iTundes don’t count towards it: they’re all in the cloud anyway, so it just lets you download them again whenever you need.

Apple has been talking about “post-PC” devices for ages, but finally, it’s actually delivering them. Between wireless updates and its new cloud back up service, you no longer need to own a PC or Mac to use an iPad, iPhone or iPod touch. For the first time, a tablet becomes a viable alternative to a laptop for many people, not just complementary.

What about Siri?

Alas, for now, the most exciting feature of iOS 5, the voice controlled personal assistant Siri, is an iPhone 4S exclusive. We doubt it’ll be heading back to many older devices ever, but one obvious candidate is the iPad 2, which uses many of the same innards, including the dual-core A5 processor – although Apple has kept mum on a possible appearance on its second gen slate. Want to talk your tablet? Don’t rule it out.

How do I get it?

You’ll need a cord one last time: plug your device into your computer and boot up iTunes when the update goes live, likely around 5-6pm this afternoon UK time. You’ll need to update iTunes to the latest version, 10.5, first though: it should prompt you to if you haven’t, but if not choose “Check for updates” on the settings.

What can I get it on?

iOS 5 will run on the iPhone 4S (duh), iPhone 4, iPhone 3GS, iPad, iPad 2, iPod touch fourth generation and iPod touch third generation. Just watch out with the latter though: that’s only the 32GB and 64GB models, not the cheaper 8GB version, which is essentially the same as the second-gen model.

Won’t it be rubbish on the iPhone 3GS?

Apple generated a lot of ill will by releasing a sluggish, stripped down version of iOS 4 for the iPhone 3G last year, but 3GS users shouldn’t be worried about the same thing happening this time: beta users have been reporting that just about every crucial new feature in iOS 5 works just fine on the 2009 model, including the Notification Centre and Newstand. About the only things that don’t? Image editing and visible tabs in the Safari web browser. We can live with that.