Block encapsulates a standalone entity that is meaningful on its own.
While blocks can be nested and interact with each other, semantically they remain equal; there is no precedence or hierarchy.
Holistic entities without DOM representation (such as controllers or models) can be blocks as well.
At the top of the file there should be a short introduction and/ or overview that explains what the project is. This description should match descriptions added for package managers (Gemspec, package.json, etc.)
Show what the library does as concisely as possible, developers should be able to figure out how your project solves their problem by looking at the code example. Make sure the API you are showing off is obvious, and that your code is short and concise.