This policy changed over time, but for a while, the only way to get a third-party web browser on the iPhone was to use an "unauthorized" service like Cydia for jailbroken iPhones.
This application was once banned and only available to jailbroken handsets, but Fring is now available through the App Store.
While Apple certainly doesn't endorse the practice, it is possible to access banned apps on a jailbroken iPhone.
In short, Cydia is a repository of applications that can only be installed and run on jailbroken iPhones.
These unauthorized applications will only run on an iPhone that has been "jailbroken."