Many people want to know what Jailbreaking your iPhone is, and whether or not they should do it. Simply put Jailbreaking allows complete customization; editing the actual coding of the iPhone itself. Apps on the app store do not allow any editing of the actual phone. Jailbreaking does, however, void the warranty of the iPhone.
The real question is should the iPhone be Jailbroken, and is it really worth it? There are numerous upsides to a Jailbroken iPhone, for instance the complete customization. The background can be changed to just about anything, so there is no more blackness. Even the icons can be changed, a user can download new text message tones, basically everything can be customized, there is no limit to what a Jailbroken iPhone can do. There are also a myriad of apps that can be download for a Jailbroken iPhone, that are free and cost money.
There are also a lot of downsides to a Jailbroken iPhone as well. Fist off there are security issues, which allow the iPhone to be hacked. Many Jailbroken apps eat up a lot of memory, so the iPhone does take a performance hit, especially on the 2g and 3g iPhones. After a while the phone just doesnt seem to work right, for instance when receiving a phone call the phone will not answer, or when answering a call you cant hear anything on the other end, but the caller can hear you. Jailbreaking does seem to make certain features on the iPhone extremely buggy.
When Jailbreaking an iPhone the user must be fairly tech savvy, and must be able and willing to troubleshoot issues, along with putting up with a few bugs. I would not recommend Jailbreaking the average users iPhone, it will just lead to frustration. Bottom line, dont Jailbreak your iPhone, it takes the phone out of iPhone.