You've been infected by a piece of Apple marketing jargon. The so-called retina display is the high-pixel-density screen on the Apple iPhone 4. It's actually at 326dpi, or 326 dots per inch. That's pretty darn good when you consider that a typical computer screen is at about 100dpi and good printers start at 300dpi. The result? Crystal clear images and - more visibly - text on the screen.

Here's another way to think about the retina display: The iPhone 4, with its small screen, has about 80% of the pixels of the far bigger Apple iPad display. Again, it's all about density. If you understand it, the iPhone 4's retina display offers an 800:1 contrast ratio, but I'm not really sure what that means, personally.

Apple describes it thusly: "the Retina display's pixel density is so high, your eye is unable to distinguish individual pixels."

Now, why are you seeing apps updated for the retina display? Because icons, graphics, and other app elements can only take advantage of the higher resolution display by being tweaked and, typically, having higher-density graphic images included in the app. Therefore what you're seeing are developers updating their iPhone apps to look better on the iPhone 4's screen. Since you're asking about it, I am imaging that you're running on an iPhone 3 or older. No worries, for you I think the apps will just prove to be a tiny bit bigger, but they'll all still function - and look - the same on your iPhone.

Hope that's helpful!