What does unlocked really mean?

I want to get an iPhone but without buying a data plan, so I was considering finding an unlocked iPhone and activating it on my plan.

However, I'm realizing that I don't exactly understand the concept of "unlocked"… LOL.

So can someone help me out and explain what it means to have an unlocked iPhone?
And jailbroken. I don't get that either.

Well unlocked basically means u r not tied to any network and you r free to change your cards. If your iPhone is locked to for example to AT&T u won't be able to use it any other network! And jailbreak means that you basically install a software "redsnow" or some else Into your iPhone via your pc and then you have to get another software called "cydia" which is just like App Store but the only difference is that all the apps are free but the only problem is that after jaibreaking your iPhone its no more safe. It can be dead soon!

Unlocked simply means that you can use it with carriers other than the one that originLly offered it. For example, the iPhone was originally offered by AT&T, but if you get an unlocked iPhone you can use it with Verizon wireless, sprint etc. Though make sure that the phone is compatible with your carrier before you purchase.
Unlocked phones can also be used A's prepaid phones. You cans simply remove the SIM card from your prepaid phone and insert it into the unlocked phone and you have a nicer prepaid option.

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