There are two types of jailbreak methods for the iPhone. A tethered jailbreak and an untethered jailbreak. Usually a tethered jailbreak comes first after a brand new iPhone model or bootrom is released, and then it is followed by an untethered jailbreak.
Although both jailbreak methods do the same thing, understanding the differences between the two types is very important to avoid any future inconveniences. The ultimate goal for any iPhone jailbreak is to install Cydia, and be able to install iPhone hacks and do many iPhone system tricks.
In this page I will clarify the differences between the two types of iPhone jailbreaks available, and will show you the advantages and disadvantages of each jailbreak type.
Tethered iPhone Jailbreak:
If you have a jailbroken iPhone and you are able to restart it and reboot it without a problem. Then, you are using an untethered jailbreak.
A tethered jailbreak is exactly the same as an untethered one, but with one inconvenience. It is the inability to maintain your jailbreak after a hard reboot of the iPhone. You must connect your iPhone to your computer every time you reboot the iPhone.
If your iPhone battery dies which requires a reboot, you will have to connect your iPhone to your computer and run the same utility that you used to jailbreak it with, and run it again. You will need a computer nearby that has the tools installed.
It doesn't mean that you have to restore your iPhone or loose any of its data. All the installed Cydia application will stay installed. All what this means is that the iPhone firmware can't be verified as clean because it is modified and therefore it needs to connect to iTunes to download or restore to a good firmware!.
The same tool that is used to jailbreak the iPhone is used again to trick it into thinking that the firmware is actually clean and not modified. Once it is verified, the iPhone will continue booting normally.
It is not possible to continue the booting process without this verification. If you connect to iTunes, you will be asked to restore or download the latest firmware which of course must be avoided.
Those jailbreak tools such as Blackra1n for iPhone 3GS or Limera1n for iPhone 4 use a simple button that you click when you connect to your Mac or PC to re initiate the jailbreak.
The button simply says says "Make it Ra1n" or other depending on what tool you used. It only takes a few seconds after that for the iPhone to come back to the home screen and to exactly where you were. Blackra1n and Limera1n were both tethered iPhone jailbreak tools.
Why Tethered iPhone Jailbreak?
Most of the time you don't have an option to whether choose tethered or untethered jailbreak. It is what is available at the time of either a new iPhone release or a new firmware or iOS.
Since a tethered jailbreak depends on the bootrom version, two iPhones could have different bootrom versions. Therefore, one iPhone could have a tethered jailbreak while the other same iPhone model could have an untethered jailbreak just because it has an older bootrom version.
Apple updated the bootrom starting with some iPhone 3GS models in November 2009 and tethered vs untethered jailbreaks started to appear. Therefore, it mostly depends on what bootrom version you have which could usually be identified with the iPhone serial number.
After the iPhone dev team gets enough time to find a good exploit in the new bootrom, an untethered jailbreak becomes available later. This has been the case since the the introduction of the iPhone 3GS. iPhones that are known to have the newer bootrom's are some iPhone 3GS, iPhone 4 , iPod Touch 3rd and forth generation, and iPad.
Advantages of Tethered iPhone Jailbreak:
1- A tethered jailbreak is diffenetly better than no jailbreak at all. You will have most of the features of an untethered jailbreak, and Cydia applications and hacks. You could think of it as a quick and short term counter measure to a permanent untethered jailbreak.
2- All your installed Cydia hacks and apps will stay installed, and all your contacts and settings will remain with no problem.
Disadvantage of a Tethered iPhone Jailbreak:
1- If your iPhone runs out of battery, you will be stock with a dead useless phone even if you try an iPhone charger to charge it. You must have a computer close by and connect your iPhone to a PC running the jailbreak tool you used initially to jailbreak.
This of course could be very annoying and difficult. It could happen when battery dies, a severe iPhone application crash, or an iPhone freeze or hang.
2- If you reboot your iPhone for any reason intentionally or unintentionally, you must connect your iPhone to a Mac or PC in order to pass the iTunes check screen.
3- You must be very careful when installing iPhone hacks from Cydia when you have a tethered jailbreak, as some iPhone hacks will require a reboot after installation. Therefore, make sure you don't play with Cydia apps unless you are around a Mac or PC with the right jailbreak software installed.
It is best to have an untethered jailbreak for your iPhone if available. However, a tethered jailbreak could be a sweet saver when a new iPhone comes out and no jailbreak is out for a long time.
So, it is still better than nothing, and usually a tethered jailbreak is shortly followed by an untethered jailbreak.
If you already have an untethered jailbreak. Then, it is recommended to stay with your current untethered jailbreak and not update to any newer iOS if:
1- There is no jailbreak for the newer iOS.
2- There is only a Tethered jailbreak for the newer iOS.
Some dongles were developed to over come the problem of needing a computer to connect to the iPhone to pass the iTunes screen. Those dongles could be a good solution for someone who travels a lot. However, they could be expensive and avoided.
If you have a tethered type jailbreak, just be cautious and always have an iPhone battery charger around you. Moreover, you should avoid at all time installing Cydia apps while outside or traveling to avoid any unexpected iPhone reboot.