What's the definition of a Retina display?
A Retina display is more of a marketing term than a precise technical term, but there is a definition: it refers to a device screen with a high enough pixel density that the human eye can't make out individual pixels - or a general 'pixellation' effect - at all. In other words, the human eye is scientifically incapable of telling the difference between a photo of a painting shown on a Retina display, and the painting itself - in theory, anyway.
Retina displays are proprietary to Apple, too - so while a rival company could produce a screen to the same specs, it wouldn't be referred to with the same word. It's an Apple-trademarked term.
My eye won't be fooled.
Perhaps not. But the scientific consensus generally backs up Apple's claims. It has been suggested that people with better than 20/20 vision might be able to pick out the pixels, but we've yet to hear from someone with that experience.
What resolution does a Retina display have?
That varies. The key factor is the pixel density, not the overall number of pixels. If you spread the same number of pixels across a larger screen, it will obviously be easier for the eye to pick out individual pixels.
And even in terms of pixel density there isn't a single figure that qualifies as Retina, since the equation also takes into account the distance of the screen from the eye. The required pixel density for each type of computing device - smartphone, tablet, laptop - is calculated based on a typical viewing distance. If you hold your iPad right up next to your face (you shouldn't do that, by the way) you may find that you can pick out pixels after all. Don't expect a refund.
The pixel density figures are: 326ppi (pixels per inch) for smartphones, held the closest to the face; 264ppi for tablets; and 220ppi for laptops.
Are there better screens than Retina displays?
Yes, but rarely - and it's debatable whether they really are 'better'. Higher-resolution screens are available, but innovation in that direction is understandably slowing down; after all, what's the point in making a screen sharper if the human eye is no longer noticing any difference? Apple's best screens are all branded as Retina.
How do Apple's non-Retina displays compare with its Retina displays?
Obviously this depends on the resolution, pixel density and so on of the non-Retina display, but it's a fairly safe bet that Apple will never sell a computing device with a fuzzy or unclear screen.
If you compare non-Retina and Retina iPad displays you can see there is a difference, but the non-Retina display is still good. If you hadn't tried a Retina display, you'd probably think it was great.
Non-Retina (left) and Retina iPad displays: a small but noticeable difference on text
The main difference is noticeable on fine detail and text. But you will occasionally be able to pick up the pixellation effect - only slightly, but it is there.
Which Apple products have got Retina displays?
Generally Apple will make this clear - the Retina display is a big selling point, so it usually puts the word Retina right there in the product name. If you don't see Retina mentioned, it's probably not there.
But here are the product areas where Apple offers Retina displays, alongside any non-Retina alternative(s):
All iPhones currently available have Retina displays. The concept was introduced with the iPhone 4, and is also present on the iPhone 4S and iPhone 5. The iPhone 5S and/or iPhone 6 are sure to have them too; it's just conceivable that a budget-focused iPhone 5C might not, but we think Apple would make savings in other areas.
MacBook Pro laptops
Is the Retina display worth the extra money?
For certain tasks, it absolutely is. If you're going to use your iPad for reading ebooks, for instance, you'll really benefit from a sharper screen. And if you edit photos on your laptop, they'll look a lot better in Retina form.
Bear in mind, too, that the price differences above don't just reflect the inclusion of a Retina display. The iPad 4 has a far more powerful processor that the iPad 2, for example, so it's much more future proofed for handling apps and games in the future.
Check out the individual reviews for more information, and since eyesight isn entirely personal thing, we'd recommend going into an Apple store, or checking out a friend's iPad 2 and/or iPad 4 (side by side if possible) to see the difference for yourself. You may not even notice one, in which case your buying decision just got easier.