If you’ve ever tried a little iPhone development out, you might have run into an inconvenient problem. Apple uses a code signing system on iOS devices to ensure that software that ends up on them has either passed through the App Store (and has thus been checked for malware-like behavior) or has been assigned to the unique device ID using an ad-hoc distribution. This is generally good for the end user, but it’s a real pain to distribute betas.
I’m not sure how it does it, but TestFlight takes the pain out of iOS beta distribution. You just build an app, upload it, enter some email addresses, and TestFlight magically takes care of the rest.
When a beta tester gets an email from TestFlight, they register their device by logging into the TestFlight website on their iOS device. It installs a provisioning profile over the air, and lets them access your uploaded application builds in the same way.
However it does its magic, TestFlight is an indispensable tool for developers.