Engadget:

AltStore works by fooling your device into believing that you’re a developer sideloading test apps. It uses an app on your Mac or Windows PC to re-sign apps every seven days, using iTunes’ WiFi syncing framework to reinstall them on your device before they expire. You only need a free Apple ID (a throwaway will do) to install apps that Apple would never allow, such as Testut’s Delta emulator for Nintendo consoles.

One of the advantages of the App Store is the vetted apps. I feel 99% safe in downloading any app from there. This is going to be the wild west of unregulated and potentially harmful apps for your iOS devices. I wouldn’t go near it with a ten-foot pole.