Where does Ruby end and Rails begin?

Rails has a huge surface area, and most people's first encounter with Ruby is through Rails, so it can be easy to forget they are entirely separate entities. Worse, Rails does actually extend a lot of what the core language provides.

These extensions are all provided through the Active Support Core Extensions library. Familiarizing yourself with it will help understanding how Rails works, reveal some of the Rails developers habits, and probably reveal a few tricks you didn't think to ask for.

http://guides.rubyonrails.org/active_support_core_extensions.html