I‘d say Rails is a combination of framework and libraries.

Some parts (ActiveSupport, the part that simplifies e.g. date calculations) are clearly libraries. They can be used outside of Rails as well, because they totally ignore the shape of your other code.

Other parts (ActionPack, the component that calls your code depending on the incoming web request) are working like a framework. Your code must adhere to their requirements, otherwise it won‘t work.