Difference between Tethered Jailbreak and Untethered Jailbreak
If somebody wants to get full administrative access to Apple devices such as iPhone, iPad, iPod Touch and Apple TV, you will need Jailbreaking. It is a method to get root access in Linux/ UNIX platform and administrative access to Windows OS. When an Apple is access through Jailbreaking, it works with any carrier be it Verizon or anyone else while working as normal as before. SIM unlocking is one of the functions of Jailbreaking. Apple dealers also sell unlocked iPhones to facilitate its customers. It also provides SIM unlocking features for a certain period of time to customers who are going overseas. For this, you just need to call carrier support center.
It is to be noted carefully though, you can call the support center and ask for Jailbreaking iPhones, yet the company Apple, itself does not recommend the process.
While in the process of Tethered Jailbreak, every time a device restarts, it needs to be connected to computer. On each step of Jailbreaking and device restarting, it needs to be connected to Jailbreak application running on the PC to resume function.
On the contrary, in Untethered Jailbreak, the Apple devices need not to be connected to PC except when the process of Jailbreaking is about to initiate. Even if the battery discharges in between, you can charge it and reboot the device. But prefer to charge the battery before you start to jailbreak an iPhone.