Cydia is an alternative to Apple's App Store for "jailbroken" devices, at this time including iPhones, iPads, and iPod Touches, specializing in the distribution of all that is not an "app".

Cydia is not available in Apple's App Store, nor is it a website: it is installed on your device using a "jailbreaking" tool, such as Pangu or TaiG.

For help with jailbreaking, see /r/jailbreak on reddit.