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

Internal Testers must be assigned the Admin, Technical, App Manager, Developer, or Marketer role in iTunes Connect, and you can only have 25 of them. You should review the iTunes Connect Roles. If you don't trust someone with these privileges, they aren't "Internal", they're "External".

You can have many more External users, but apps need to go through the App Store review process before these users can test them:

Apps made available to external testers require a Beta App Review and must comply with the full App Store Review Guidelines before testing can begin. A review is required for new versions of your app that contain significant changes. Up to 10 apps can be tested at a time, internally or externally.

Upload your app to TestFlight

Both internal and external testers will install your app from the TestFlight app. Once invited, they will be sent an email asking them to install the TestFlight app. Once they have done so, they'll be able to install your beta app...

Internal Testers: Think of these users as employees who receive instant updates to your app without approval/review

• Must be added manually via iTC
• 25 Max allowed
• Once your app is uploaded it's available immediately for internal testers (before it has been reviewed)
• All internal testers must be added as a user in your iTC "Users and Roles" settings, which gives them certain permissions (review other answers and the docs for this). You wouldn't want to give just anyone permissions here.
• Do not have a 60-day time limit

External Testers

• Will only be able to use your uploaded build for up to 60 days. If you add additional builds, they can update, and the 60 days starts over again.

• Will be able to test your app after

  1. You have submitted it for review
  2. It gets approved in TestFlight review and
  3. You set it to be available for testing. The review process us usually instant for new builds with the same version number. If you add a new version number, the review process can take up to 48hrs as of 10/2016.

• Can be added automatically by importing a csv file or with Fastlane's pilot automation tool.

• As of now (August 2016), up to 2000 email addresses can be added. Each email address will allow a user to install the app on multiple devices. The email addresses do not need to match their Apple IDs.

• They receive an invite to install your app once your first build is available for testing. If you add a new user after making a build available for testing, they'll immediately receive an invite. All users will receive notifications to install newer versions of the app if you upload additional builds.

• Will be disallowed from using your app after you have pushed it to the official app store (which promptly ends the beta) or 60 days have passed since you started the beta, whichever comes first. If you end the beta without launching in the app store, and they try to open it, it will crash. Yay, Apple UX! If you do push a version to the app store with the same bundleName, version, and bundleID (build number doesn't matter), then your beta testers will automatically receive the app-store version of the app when it goes live.

Internal Testester

1. Email id should be used with apple account
2. Internal tester will be a member of developer account. Roles may beAdmin, Technical, Manager, Developer, Marketer
3. Only 25 internal testers are allowed
4. Each tester can test on upto 30 devices
5. Can test 100 apps at a time
6. Build remains active for 90 days
7. No app beta review

External Tester

1. Email id need not to be used with apple account
2. 10,000 external testers are allowed
3. Can test 100 apps at a time
4. Build remains active for 90 days
5. App required beta review.