Cydia is a replacement packaging and repository manager for the original Installer.app for the iPhone or iPod touch. It was created by Jay Freeman to replace BSD Subsystem and Installer.app.To get Cydia you need to have installer.app already.Cydia’s repositories allow the installation of software packages, most of which are available to download at no cost.Cydia also includes the Cydia Store, which offers a number of applications for sale.
Cydia itself looks similar to Installer.app but add many improvements to organization, functionality and performance of Installer.
With Cydia you can see the number of apps in each category,see details of what installing each app include such as file size and functionality added,search for apps by name,see changes to your currently installed apps and available upgrades,see details of currently installed app and easily manage them.
You can share your iPhone’s 3G connection with your laptop using MyWi, or get notified in your status bar of new mail and texts using StatusNotifier.
Cydia is not required to install APT packages; however, it is greatly encouraged by most users as it provides a much easier way to find, install, and manage packages.
In addition to offering applications to install, as of September 2009, Cydia was further improved to cache the digital signatures known as SHSH blobs used by Apple to verify restores of the iOS firmware. This gives users the benefit of being able to downgrade their firmware to a previous version by means of a replay attack.