Cydia is an installer quite similar to App Store which will permit you add different sources to install third party apps. You can find quite everything, particularly some apps denied by Apple on its App Store.

If Cydia crashes :

• Connect to your iPhone in SSH and go to var/lib/apt/lists
• Remove all files in this folder except partial
• In the folder partial, remove all files.
• Respring and start again Cydia.