Can anyone tell be in one or two sentences how this relates to Scratch? Is is an alternative, is it used by Scratch, is it something different although it looks very similar?
It's based off Scratch's UI but doesn't share any code. It has a lot more features that delve more into computer science instead of scripting like in Scratch. I believe it was also available in-browser before Scratch was, but I'm not sure.
Snap! is designed to allow students write code in the same way they might in Scratch, but then move them on to exploring CS concepts. It's also somewhat more extensible, including allowing people to make blocks pseudo-natively in JS. (We don't really expect students to do this, but it's a great way for instructors to build tools for assignments or just experiment with ideas not yet in the language.)
The first version, BYOB, was an extension of the Scratch 1.4 source code, but since then we have moved in different directions. But we are still inspired by how careful they were in designing visual metaphors for computer science ideas and have tried to extend that care to things like lambda and object inheritance.