SP: English Study Group

The firm has also done a deal with publishers including Penguin, Macmillan and Harper Collins to allow e-books to be downloaded directly to the device through a new iBook Store.

"You can download right onto your iPad," said Mr Jobs.

He also showed off magazines and newspapers on the device.

'Gold rush'

He told an audience of journalists, analysts and industry peers that the device lets people "hold the whole web in your hands".

"What this device does is extraordinary. It is the best browsing experience you have ever had," he said.

The device has a 9.7-inch multi-touch display, allowing people to type directly on to the screen, as well as manipulate pictures and control the action in games with their fingers. However, users can also plug in a keyboard.

Apple claim it has a battery life of 10 hours.

It comes preloaded with twelve applications - essentially multi-touch versions of existing Mac software such as iPhoto.

However, owners can also download third party apps - both specially designed for the iPad and those already available for the iPhone. People with both can synchronise their apps between the two devices.

"We think it's going to be a whole other gold rush for developers," said Scott Forstall, who runs Apple's app division.

Apple revealed that more than three billion apps have been downloaded from its App store.

The New York Times showed off its app for the iPad, which recreates the look and feel of the newspaper but allows it to have new features, such as video.

"We're pioneering the next version of digital journalism," said Martin Nisenholtz, a senior executive at the newspaper.

It also includes the firm's iTunes software built in, allowing people to purchase songs and movies straight to the device.

'Cheap laptops'

It is not the first touchscreen tablet computer on the market. Earlier this month, manufacturers such as Dell and HP showed off devices at the Consumer Electronics Show (CES) in Las Vegas.

Some industry experts have questioned the need for another category of device, alongside laptops, smartphones and netbooks.

Analyst firm CCS Insight said that it remained "sceptical" of the market. It described the iPad as "a supersize iPod Touch that would get little interest if not from Apple".

Mr Jobs dismissed netbooks as "just cheap laptops".

"Netbooks aren't better at anything - they're slow and have low quality displays," he told the audience.

"They're not a third category device, but we have something that we think is."

The cheapest iPad, which will come with 16GB of flash memory and wi-fi will cost $499. The most expensive version, with 64GB of storage and the ability to connect via a mobile 3G signal, will cost $829.

Buyers of the versions with 3G connectivity will also need to sign up to a data plan with a telecoms provider, but Apple's US carrier AT&T said it will be offering subscriptions on a monthly, not annual basis.

CCS Insight said the high cost would put it "beyond most consumers".

Mr Jobs said that he hoped to have international prices in place in June or July.

However, all the 3G models are unlocked, meaning they will work with any network.

The launch puts to rest months of speculation and rumour in the blogosphere.

Apple - famous for its secrecy - had remained silent in the run up to the launch, unwilling to release any details publicly.

The iPad is in effect a giant iPhone which can do just about everything the phone can do but may provide a better way of watching video or playing games.

The most interesting aspect is the launch of iBooks, the online book store with which Apple hopes to revolutionise the publishing world just as iTunes transformed the music industry.

That could spell trouble for Amazon's Kindle and other e-readers.

But the big question is whether Steve Jobs is right in thinking there's a yawning gap between smartphones and netbooks which the iPad will fill.

It's not entirely clear if a huge number of people - apart from dedicated early adopters - are desperate for yet another device.

iPAD SPECIFICATIONS