If you just bought your new iPhone or iPad, then first thing you might hear from friends will be; 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 install Cydia on iPhone or any iDevice in an easy way.
You will need your iPhone and wifi internet connection to install Cydia on iPhone.
JailbreakMe is the easiest way to free your device online. Experience iOS as it could be, fully customizable, themeable, and with every tweak you could possibly imagine. Safe and completely reversible (just restore in iTunes), jailbreaking gives you control over the device you own.
Open safari on your iPhone and go to:
There click on the icon that says free and install it. Let the installation proceed and there you go. Cydia has been installed on your iPhone successfully.
Tip: After installing Cydia on iPhone, next you should install the Repos or Sources in Cydia.
UPDATE: jailbreakme.com was for older versions of Apple devices and it does not support recent firmware.