Originally Posted by tHeFiRsTiDioT
Cydia is an installer for non-appstore apps - it needs a jailbroken iphone
Cydia allows you to install apple unapproved 3rd party applications onto the iphone easily. Think of it sorta like the App Store for apple unapproved apps.
If you jailbreak your iPhone / iPod touch with e.g. QuickPwn you will automatically be asked if you want Cydia installed.
While Apple still has not approved a video recorder for the iPhone, Apps running in the background, Offline Wikipedia in other languages than English, copying, moving or deleting files via SSH or FTP,... all this and much more can be found in Cydia. It can be installed within minutes.
If someone is interested just google for "iphone dev team" and search their blog for "quickpwn".
Quick and very simple stuff. But be aware that there still is the possibility of bricking your beloved Apple handheld.