Cydia is a Debian APT package manager that lets you download tweaks and apps that haven't been approved by Apple for your jailbroken iPhone, iPad and iPod Touch, via repositories. Repositories with additional packages can be added to Cydia through "Manage->Sources->Edit->Add". Cydia is preinstalled with most iOS jailbreaks, since iPhone OS 2.0. Cydia for iOS 11, updated by Saurik, is officially supported in unc0ver by @Pwn20wnd.
On December 13, 2018 Saurik has announced that the Cydia Store is shut down because of a bug with Paypal and lack of funds. He recommends to remove the saved Paypal info from the Cydia account and assures the users that he intends to maintain the ability to download existing packages. https://www.reddit.com/r/jailbreak/comments/a5wfq9/news_andrew_wiik_recommend_that_everyone_removes/ebpur5a/?context=1
- the user can add and remove sources (repositories) to check for apps
- Cydia is iOS alternative cuz when you jailbreak your iOS you will have cydia
- This is not really for apps, it is more for tweaks to your device, but does not provide many actual apps.