What is the Cydia iPhone app?

Cydia provides an alternative to the Apple App Store on jailbroken iPhones. The app can be used to download and install a variety of software packages that add functionality and customize iOS devices. Cydia consists of free software packages and the Cydia Store, where apps can be purchased.

Cydia is named after the codling moth (Cydia pomonella). Larvae of the codling moth are otherwise known as apple worms. After three weeks of eating the insides of an apple, the worm moves on and later develops into a moth.