Difference between internal and external testers in iTunes Testflight beta testing?

Question

Can anyone please explain in complete details the difference between internal and external testers in iTunes Connect Testflight beta testing? The difference here is just too

Answer 1

iOS app testing for getting early feedbacks may be done in multiple ways. Enterprise, Adhoc and the new iOS8 Testflight Beta testing

Ad-hoc Provisioning

In this method the focus is on devices rather than the users. The devices UDID need to be registered in member center and the deployment profile MUST have the UDID of the specific device to download the app build. The app may be hosted on testflight or even dropbox (Only https: is supported).

TestFlight Beta Testing for iOS applications:

UDID is no longer required. The focus is on test users rather than device. A single user may have multiple devices on which the beta app may be installed.

For the Test Users of Beta Testing please refer to the "Aaron Brager's" Answer.