Apple needed to go big this time, and it did. The iPhone 5 represents the most radical revision yet to the biggest-selling smart phone: It has enough improvements and enhancements that even satisfied owners of the last-generation iPhone 4S will want to consider upgrading.

Here's what's most important about the new phone, which is available to pre-order on Friday—from AT&T, Sprint, and Verizon, at $199 and up—and ships on September 21.

A bigger, wider screen. Every iPhone until now has had a 3.5-inch screen, even as competitors' displays grew and grew. The iPhone 5's 4-inch screen is still smaller than the 4.5-inch-plus displays of the biggest smart phones. But the 1136x640-resolution Retina display will retain the same sharp 326-pixels-per-inch resolution of its predecessor, the iPhone 4S.

In addition, the display's taller, slimmer 16:9 aspect ratio will make it better for viewing wide-screen video and game content. And rather than stretching and distorting the square images of older apps and other content to fit across the screen, the phone will render them optimally by centering content on the screen in full sharpness, with black bars on the side for 4:3 images. Apple promises 44 percent better color saturation and says that touch is integrated into the display, rather than as an additional layer, helping to make the screen thinner and more glare-resistant.

A thinner, lighter presence. The iPhone 4 and 4S are chunky—fairly thick, and among the heaviest smart phones in our Ratings. By contrast, at around 20 percent thinner and lighter, the iPhone 5 will be among the more slender and lightweight of smart phones.

Faster data connections. With the iPhone's key competitors all offering high-speed 4G wireless connectivity, it isn't surprising that Apple added to the iPhone 5—including to LTE, the fastest flavor of 4G. The new phone will also boost Wi-Fi speeds over Apple's previous maximum of 150 megabits per second (Mbps).

Video calling over wireless networks. For the first time, iPhone (and also iPad) users will be able to use Facetime, Apple's video-calling app, over cellular data networks rather than only on Wi-Fi. (While that will add convenience, the data-hogging bandwidth required by video calls could also send monthly wireless bills soaring, unless you're careful.)

Siri will get appy. For the first time, Apple's voice-recognition assistant will be able to launch Facebook and other third-party apps and games on command—allowing you, for example, to use Twitter and compose Facebook posts with her help. The iPhone 5 also has three microphones, and Apple says it will provide better noise-canceling when you speak into, it, promising to allow those you call—and Siri—to understand you better.

An even-better camera. iPhone's already top-notch camera has been upgraded, promising shorter shutter lag and better low-light performance, while adding such popular Android-camera feature as panoramic stills.

What's up, dock? One possible sour note for long-time Apple fans may be that Apple replaced the standard 30-pin Adapter USB adaptor built into all iPhones, iPads, and most iPods—and the plethora of accessories they dock with—with a much smaller (80 percent smaller) connector called Lightning. The smaller 8-pin design of the Lightning connector puts its four electrical contacts on each side, so it doesn't matter which way you plug your device into it.

Apple will offer a Lightning to 30-pin Adapter for $29 in October that will enable the iPhone 5 and new iPods to connect to "vintage" boom boxes and other accessories. But video and iPod out will not be supported. No word yet on an adapter to let owners of older iPhones and iPods connect to the new accessories expected soon with the new Lighting docks. [Updated]

Bottom line: A lot has changed in smart phones in the year since the iPhone 4S was announced. The iPhone 5 represents a significant jump from that phone, which represented only a modest upgrade from the iPhone 4, which launched in mid-2010. We're eager to see how the iPhone 5's host of notable upgrades and innovations will play out in our labs—where we will be testing the new phone, beginning on its September 21 availability date.