AltStore is an alternative app store for non-jailbroken iOS devices.

AltStore allows you to sideload other apps (.ipa files) onto your iOS device with just your Apple ID. AltStore resigns apps with your personal development certificate and sends them to a desktop app, AltServer, which installs the resigned apps back to your device using iTunes WiFi sync. To prevent apps from expiring, AltStore will also periodically refresh your apps in the background when on the same WiFi as AltServer.