After getting an iPhone or iPad, first thing users usually hear is Cydia and second will be Installous. Cydia is app for iOS that enables user to find and install software packages (apps, interface customizations, system extensions) on a jailbroken (factory unlocked) iPhone, iPod Touch or iPad. Most of the apps available through Cydia are free but it also includes apps for sale through the Cydia Store. Most of these packages focus on providing customizations and modifications (tweaks) that can only run on jailbroken devices. To install Cydia you must jailbreak your iDevice. Here is how to jailbreak iPhone and install Cydia in simple way.
The iDevices (iPhone, iPod touch and iPad) hacking that allows users to access to the entire Unix filesystem is called jailbreak or Jailbreaking is a process that gives a user read/write access to locked OS files on iDevices. Jailbreaking gives a user complete control over iPhone (and other iDevices) including customizing it and 3rd party apps installation. Jailbreaking is a hack to gains access to areas of the iPhone, iPod touch or iPad that users are not supposed to mess with. After that users can install 3rd party cool programs or unlock the handset for use with another cellular network. Lets talk about jailbreaking in detail.
Comex, the developer of JailbreakMe 3.0 is about to release the iPad 2 jailbreak. One of the beta testers of iPad 2 jailbreak via JailbreakMe has leaked the jailbreak. He was able to jailbreak iPad 2 by downloading a pdf file in mobile Safari on the iPad 2 which contains the exploit developed by Comex and will jailbreak your iPad 2, allowing you to access Cydia. This iPad 2 jailbreak is currently in beta version and will contain few bugs and will only work on WiFi models of the iPad 2. Safari has been reported to crash on your iPad 2 if you use this method. Better approach is to wait for Comex final official release.