Jailbreaking your iPod or iPhone allows you to install app from other sources different from the app store. Also you can modify the look and feel of the phone in ways you can't with the stock software. And you gain access to the file system of the phone. It will void your warranty tho.
When you jailbreak your iPod or iPhone, you are in a position to get various application. This is because by doing so, you will have unlocked the operating system or firmware. However, when one jailbreaks, you have to ensure that you have done it safely.