purplera1n is a utility that jailbreaks version 3.0 and up of the iPhone OS

IPhone OS

iOS is Apple's mobile operating system. Originally developed for the iPhone, it has since been extended to support other Apple, Inc. devices such as the iPod Touch, iPad, and Apple TV. Apple, Inc. does not license iOS for installation on third-party hardware...

. It achieves this by patching the firmware of the device "on the fly", meaning it edits the firmware located on the device whilst in DFU (Device Firmware Upgrade) mode. It allows users to install either Cydia or Rock App, a Cydia alternative. This allows the user to gain access to the root directory

Root directory

In computer file systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the root of a tree — the starting point where all branches originate.-Metaphor:...

 and file system

File system

A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...

. By gaining access to those, users can install themes, tweaks and other homebrew

Homebrew (video games)

Homebrew is a term frequently applied to video games or other software produced by consumers to target proprietary hardware platforms not typically user-programmable or that use proprietary storage methods...

 applications.

Purplera1n has been superseded by blackra1n

Blackra1n

blackra1n is a program that jailbreaks versions 3.1, 3.1.1 and 3.1.2 of Apple's operating system for iDevices, known as iOS.The program uses a bug in the USB code of the firmware for iDevices, allowing unsigned code to be executed. Blackra1n uses this exploit to patch the firmware of the iDevice...

.