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(CNN) -- Despite how shady and against the rules it sounds, "jailbreaking" mobile phones is now legal in the United States, according to a new government ruling.

But what does that mean? And why do people jailbreak phones?

The simple answer is this: To gain more control.

Some mobile phones, particularly the iPhone, come with restrictions on what types of apps -- or programs -- you can purchase and run, which cellular network you can use and, essentially, what you can do with the phone.

Jailbreaking the iPhone allows you to shop for apps anywhere on the internet, not just the iTunes App Store, where all of the apps must be approved by Apple to go on sale.

There are several types of apps that Apple apparently won't approv... e, and you can find many of these in a sort of black market (now legal) app store for jail breakers, called Cydia.

Here are some popular apps you can use if your phone is jailbroken, but that you won't find in the mainstream App Store (special thanks to Kyle Matthews, owner of the site modmyi.com for his advice here):

MyWi: This app turns the iPhone into a mobile Wi-Fi hot spot. That means you could jump on the internet with a laptop, even if you can't find a public Wi-Fi network to join, and don't have a 3G card.

IntelliScreen: It lets you customize the iPhone's home screen, which normally just shows the time, date, and a switch that lets you open the phone. With this app, however, Matthews gets his e-mail and calendar on that home screen, so he doesn't have to open the phone to figure out what's going on.

Tlert: Instead of having to open the iPhone's text message program, this app lets you respond to new text messages from any program or screen.

Perhaps the most popular reason for people to jailbreak their iPhones is so that they can "unlock" them, too.

This is a bit confusing, but there are fundamental distinctions between "jailbreaking" a phone, which lets you download any app, and "unlocking" it, which allows you to access other wireless networks with the iPhone.

To unlock an iPhone, you first have to jailbreak it. (Apple doesn't want people to stray from AT&T, which is the exclusive wireless network provider for the iPhone in the United States.) Then download an app -- for instance, Ultrasn0w -- that will open your phone to other networks.

This isn't a cure-all, however.

Unlocking an iPhone will allow it to access other "GSM" networks, which, in the U.S., only includes T-Mobile and AT&T. Even if you unlock your iPhone, you still won't be able to use it with Verizon or Sprint.

However, many international carriers operate GSM networks, so people who travel frequently may want to unlock their phones to avoid international roaming fees.

Once the iPhone is unlocked, insert a SIM card from the new network you'd like to use and then have at it. (On the iPhone 3G and 3GS, the SIM card is found at the top of the phone; insert a paper clip into a tiny hole to make it pop out. On the iPhone 4, a mini-SIM card is on the side, and you'll still need the paper clip.)

It's important to note that there are downsides to jailbreaking and unlocking your phone, even if those acts are now legal under federal law according to Monday's ruling by the U.S. Copyright Office, which is part of the Library of Congress (read a statement from the Library of Congress here).

Apple says jailbreaking the iPhone makes it more likely to crash.

"Apple's goal has always been to ensure that our customers have a great experience with their iPhone and we know that jailbreaking can severely degrade the experience," Apple spokeswoman Natalie Harrison wrote in an e-mail to CNN.com. "As we've said before, the vast majority of customers do not jailbreak their iPhones as this can violate the warranty and can cause the iPhone to become unstable and not work reliably."

Jailbreaking the iPhone also voids its warranty with Apple, so if your phone suddenly dies after it's jailbroken, Apple doesn't have to fix it.

Connecting a jailbroken phone to iTunes, however, can restore it to its original condition, according to Matthews.

"Apple can and does still void your warranty if the device is jailbroken, so you are able to at any time plug it into iTunes and press restore and iTunes will automatically restore your iPhone to a completely nonjailbroken state," he said. "So it's not something you can't go back on."

A number of free and paid programs will jailbreak the iPhone in a single click. Among the more popular ones are Spirit and PwnageTool, Matthews said.

You can find "how to" guides for jailbreaking the iPhone, iPod Touch and iPad on ReadWriteWeb, hackthatphone.com and Gizmodo.

Users who want to jailbreak their phones install those programs and then follow the instructions. It's worth noting that no one has posted a program that will jailbreak the iPhone 4, but bloggers, including Matthews, expect a hack for Apple's newest phone to be posted online within a matter of days or weeks.