Pop vs Principle: What are the differences?

Pop: The extensible iOS and OS X animation library behind Facebook's Paper. Pop is an extensible animation engine for iOS and OS X. In addition to basic static animations, it supports spring and decay dynamic animations, making it useful for building realistic, physics-based interactions. The API allows quick integration with existing Objective-C codebases and enables the animation of any property on any object. It's a mature and well-tested framework that drives all the animations and transitions in Paper; Principle: Animate Your Ideas, Design Better Apps. It makes it easy to design animated and interactive user interfaces. Whether you're designing the flow of a multi-screen app, or new interactions and animations, it helps you create designs that look and feel amazing.

Pop and Principle can be primarily classified as "Mobile Prototyping & Interaction Design" tools.

Pop is an open source tool with 19.7K GitHub stars and 2.9K GitHub forks. Here's a link to Pop's open source repository on GitHub.