Cydia is an application which consists with different types of applications, tweaks, themes, extensions, settings, features etc. It also seems as the Apple's app store, but works as third party bundled apps. So it can be simply introduce Cydia as the third party apps for iPhone and iPads. But Cydia download is not available in Apple app store. To download cydia, you must follow a process which is to jailbreak the devces or install cydia apps online through a web browser.