The first idea may come is that jailbreak lets you to crack your phone to use some application which are not allowed by Apple. Well it’s not a proper definition but somehow it fits with the logic a bit.
You want to know that how do you do that?” Jailbreaking provides you the liberty to change the iPhone’s firmware and get the access to the internals of its operating system and can install third-party applications on your iPhone that are not otherwise available through Apple.
Jailbreak lets you do everything from customizing the look of your iPhone to installing third-party applications and customized ringtones on it. Depending on how far you’re willing to go, you can do even more than that: Jailbreaking even lets you to unlock your phone so you can use it with a carrier and other service provider.
Jailbreaking process is totally safe. If anything goes wrong with your iDevice, you simple just connect with your computer and restore it using iTunes.
If you believe that by jailbreaking you wont be able to use default features/applications, then its not true. You can still use appstore, itunes, phone, calculator and everything can be used. Jailbreaking only adds an extra icon called Cydia.
It can be a confusing process if you are newbie. So its better to take backups or restore to avoid afterwards errors.