Explaining iOS 8’s extensions: Opening the platform while keeping it secure

Of all the new features introduced in iOS 8 last week, app extensions are the ones that will have the biggest, most visible impact on the new operating system. The feature is most commonly described as a way for third-party applications to talk to each other, though that's an oversimplification—that's not the only thing app extensions can do, and technically third-party apps in iOS still can't talk directly to one another.

We've read the developer documentation and watched the WWDC sessions. Now it's time to break down how these features work, what they do, and how Apple is continuing to balance third-party access to the operating system with security.