Apple’s iPhone 5 goes on sale Friday, including a polished new look. But only one new feature actually matters: Its super-fast LTE wireless connection. With Internet access that’s amazingly quicker than before, mobile computers like the iPhone 5 can finally live up to their promise.

What’s LTE?

Simply put: It’s the latest version of wireless network technology, used in the U.S. by Verizon Wireless, AT&T, and more recently, Sprint. LTE has been around for a couple of years, and Apple included it in this year’s new iPad. (That’s how I know firsthand how amazing it is.)

But this is the first time Apple is including LTE in an iPhone. This means millions of people will be using it for the first time over the next few months. You may hear the terms “4G” or “4G LTE” too, but nevermind the jargon. Just know that LTE is impossibly fast.

Why LTE Is Revolutionary

Stop me if this sounds familiar: Walking to the train station, want to load my email before I get on the train… Waiting… waiting… waiting. And, there goes my train. Or: Trying to load this map so I can find where to meet you… Loading… loading… loading… loading…

In practical terms, LTE means you’ll finally be able to download stuff to your phone fast – in many cases, faster than your home broadband connection. It means you’ll finally be able to stream video – even in high resolution – without endless buffering. Or even video chat. It means you’ll load your Instagram feed in a snap. It means never waiting for your phone anymore; it’ll finally be waiting for you.

Don’t get me wrong: Having an iPhone for four years (and a Palm Treo for two years before that) has been much better than the dinky candybar phones before. Relatively reliable access to maps, GPS, email, shopping, and entertainment has legitimately changed my life. But the lag of a slow mobile Internet connection – familiar to many AT&T iPhone subscribers! – has always added an element of frustration. I stopped trying to watch video on my phone a long time ago.

As I’ve experienced on my iPad, LTE changes this. It’s going to be as big of an improvement to the iPhone as the iPhone was to prior phones. Or, if you’ve used a computer with a solid-state (“flash”) disk, as big of an improvement as that was from old, spinning hard drives. Especially for a mobile device, speed really matters.

And don’t take my word for it. LTE got a lot of praise in the early iPhone 5 reviews. LTE “data downloads and uploads just fly,” Walt Mossberg wrote for the Wall Street Journal. “Using the iPhone 5 on LTE is nearly indistinguishable from using it on Wi-Fi,” John Gruber said at Daring Fireball. “Web pages load in a snap, Siri parses input and responds promptly.” The New York Times’ David Pogue called it “wicked-fast.”

What’s The Catch?

For one, there’s a risk that mobile operators – running LTE networks for the first time – will be in for a cruel surprise, just as they were with the iPhone 3G and their old networks. We simply don’t know if LTE networks will be able to keep up with rising traffic. AT&T and Verizon could each conceivably get 5 million or more new LTE users by the end of this year, and that might swamp their networks. We just don’t know.

The other potential catch is that LTE ends up being so good that we use it more than we ever used 3G, costing us more money. Mobile operators are switching their pricing to match the future of their business, where Internet access is more relevant than selling “buckets” of voice minutes for phone calls. If you fall in love with streaming video or music over LTE, it may be costly. Perhaps it’ll be worth it, if you’re getting a lot of value out of it. But don’t be surprised if your mobile bill goes up rather than down over the next few years.

Still, if all goes according to plan, LTE is going to be a killer feature for the iPhone 5 and for mobile computing. This stands to benefit phone makers, mobile operators, and especially application developers. Apps should work as well on mobile devices now as they do on desktop computers. And that opens all kinds of new doors.

Photo: NASA/MSFC via Flickr

Related: The Real Reason AT&T And Verizon Are Switching To “Shared” Pricing Plans