Before we find out what jailbreak does to your iPhone we must find out what jailbreak means. If you already have an iPhod to know that you can download iPhone applications from iTunes Store only. Maybe you want to have some new applications that can't be found on iTunes Stores, but you can find them on the Internet. Jailbreaking the iPhone is the process that allows you to install other application on your iPhone, beside the ones that you find on iTunes Store and if you want to undo the process you can always use iTunes to restore the iPhone default factory settings.
You should know that there is a difference between jailbreak a phone and unlock a phone. You already know what jailbreak means and know you will out what unlock means. iPhone is using only SIM card from AT&T because the iPhone is distributed only by AT&T in US. If you unlock your phone you can use any kind of SIM card you want, from any phone company (for example Vodafone or Orange).
After you jailbreak your iPhone you can install any kind of application you want your iPhone to have.
One of the applications you can tray is iPhone modem application; this one allows you to use your iPhone as a modem and after you install it you can surf the Internet from your laptop or your desktop.

If you want to record at a higher level with your iPhone you can jailbreak it using Cycorder. This application turns the iPhone camera into a video recording camera. The CyCorder application is free and gives your iPhone a better video quality and a better recording speed. This application offers you real time video compression; this can record 6-15 frames per seconds and you wont have to worry about compression delay. The application can record video but not audio, therefore you won't to able to hear what you are recording. It is very easy to use, after you install the application your iPhone will restart automatically. You will se the record button at the bottom of the screen, al you have to do it tap the button and the recording starts. If you want to stop recording just top the record button again. At the bottom-left you will see a triangle button; this one is for playing back the video. If you recorded a lot of videos, the application can make a list of them.
If you want to change the look of your iPhone you can do that with the Winterboard application. First you must install the application through Cydia and them you can install and select the themes you like. To see the theme on your iPhone go back to Home Screen and launch the application and then just tap on the one you have installed and then click Home button. You must wait for the Winterboard to load the skin, this will take just a few seconds and there you have it - your iPhone's knew look.
These are some examples of what jailbreak can do to your iPhone and if you found them interesting you may find interesting some jailbreak tools for iPhone.
One of them is Readsn0w.This tool supports the next models of iPhone: iPhone OS 3.0, iPhone 2G, 3GS and 3G. It supports also iPod Touch if you want to jailbreak it. It is the most user-friendly application; it can work both on Mac and Windows.
Another jailbreak tool for iPhone is QuickPwn. This tool is available for iPhone 3G and 2G. Both Mac and Windows versions are available for download. It comes with a built-in utility if you want to unlock you iPhone.
PwnageTool, developed by iPhone Dev Team. This tool is available only for Mac platforms. It is for iPhone 2G, 3G and 3GS. This tool creates a custom firmware file for iPhone jailbreaking. This tool is considered very flexible because it allows its users to have a direct modification of the firmware file.

PurpleraIn is a jailbreaking tool that work only for iPhone 3GS. It can work both on Mac and Windows.
If you found the tool you want for your iPhone jailbreak the next step is to find an application installer. The most common applicaton is Cydia, this application is installed during the jailbreak process. This is the application that lets you download and install the new application for your iPhone. The Cydia application is very easy to manage and use. When you launch for the first time Cydia you will be asked to select what type of filter you want. The application gives you three types of filters to choose from: Users, Hacker and Developer. If you are a normal user you can select the "user" filter. After you have chosen the filter you are in Cydia: the interface is very easy to use.
Home - in this screen you can find the latest news of this application and a user guide
Section- in this screen you will find the applications that are available for download.
Changes-this screen shows you the latest applications available for download and also shows is you have some upgrades for your old applications.
Manage screen keeps a track of your applications. You can use it to uninstall or to upgrade packages.
Search screen- in this screen you can search for application in Cydia.
Jailbreaking the iPhone is a good alternative if you want your iPhone to have the best applications available to download.

mobile online, on April 01, 2010
I don't know about you, but my Nokia e63 mobile is a champ. Even their mobile online support is great. I'll take my Nokia over an iPhone any day. As many blogs on www.dozenmobile.com say, there's a wealth of cool new phones out there. But if I want to change phone companies, no jailbreaking here, I just have to change the sim. Take that apple!!