Cydia is an alternative app store for iOS devices that allows users to download and install applications not available in the official Apple App Store. It is primarily used on jailbroken devices, which have been modified to remove restrictions imposed by Apple. Developed by Jay Freeman, Cydia provides access to a wide range of apps, tweaks, and themes that enhance the functionality and customization of iPhones and iPads. Users can explore various repositories to find software that suits their needs, making it a popular choice among tech-savvy individuals looking to personalize their devices.