Apple’s “Retina” display (a.k.a. high-DPI or HiDPI) devices have changed the way all websites can be viewed. Here’s a list of Apple’s current Retina devices’ display dimensions:
iPad 3: 2048-by-1536-pixel resolution at 264 pixels per inch (ppi)
iPhone 4 and iPhone 4s: 960-by-640-pixel resolution at 326 ppi
MacBook Pro (15″ Retina): 2880-by-1800 resolution at 220 ppi
Future device resolutions will be found at Apple’s Tech Specs site
In OS X, there are four times as many pixels for a given area on a high-resolution display. An increase in pixel density means that drawing is more precise, resulting in sharper text, extremely detailed photos, and a more defined user interface.