Jailbreak is one of the popular method to hack the iphone, ipod, ipad or apple tv.In technical terms it allows user to gain access over Unix file system of your i-Device and helps you to add additional applications through Cydia.With the help of Jailbreak user gain root access to the operating system and can add extra themes,apps and software which are not available through official apple store.
Why use Jailbreak
Jailbreak allows you to add custom themes, icons and application that are not available through official apple app store.Your  iphone will will be more powerful with the help of Jailbreak.You can add a number of free applications.
Another advantage of Jailbreak is that you can unlock you iPhone and use any other mobile operator.Here is a JailbreakWizard which tells you whether you can Jailbreak and unlock your iphone, ipod ,ipad or Apple tv.Below is a video which demonstrate the various uses of Cydia

Disadvantages of Jailbreak
Now you know the advantages of Jailbreak.Now question is - Is there any back side of jailbreak.Answer is YES.First and major disadvantage of Jailbreak is that your warranty is void.Apple easily detects a jailbreak and never allow its users to jailbreak their device.
Another disadvantage is if you stuck in the process of Jailbreak than it might take many of your valuable hours to solve the problem.I am saying this with my personal experience :-(
Important points before Jailbreak
1)First check whether you can Jailbreak your device or not.If you are trying to unlock your iphone using Jailbreak than you must check it before proceeding.This guide will help you to know whether your device can be Jailbreak/unlock or not.Click here to Read the Jailbreak/Unlock Guide
2)You must update your itune software to latest version.Older version of itunes cause problem during restore of custom IPSW files.
3)It is better to turn off the windows firewall during the jailbreak process.
4)Only use proper version of software and files for Jailbreak.A list of such software and files is provided in Jailbreak Wizard
If anything  goes wrong than you can restore iphone using instructions listed here.

If you have been trying to upgrade your iPad, iPhone or iPod Touch to the all-new iOS 5 but getting unknown errors with codes that read like 3200 or 3002, here’s one possible solution.
I have used the same technique to update my iPad 1 and iPad 2 with iOS 5 and the software installed without any problems.
First you need to download the iOS 5 IPSW files from the Apple servers. Click the link that corresponds to your iOS5 device - iPhone 3GS, iPhone 4 GSM, iPhone 4 CDMA, iPad (original), iPad 2 WiFi, iPad 2 GSM, iPad 2 CDMA. Remember that IPSW files are 700+ MB in size so you should probably use a download manager that can resume broken downloads.
Once the firmware file has downloaded, close the iTunes software and also disconnect your iPhone / iPad  from the computer. Next copy and paste the IPSW file to the following location:
On Windows:
%APPDATA%\Apple Computer\iTunes\iP{one|ad|od} Software Updates
On Mac:
~/Library/iTunes/iP{one|ad|od} Software Updates
Now plug-in your iOS device to the computer using the USB cable, launch iTunes and click Update to upgrade your device to iOS 5. This time it will directly pick the local .ipsw files and the upgrade should happen without any issues.
You can also upgrade the iOS firmware manually by holding down Shift while pressing the Update but for some reason, this approach always resulted in same 3200 and 3002 errors.