The app/controllers subdirectory is where Rails looks to find controller classes. A controller handles a web request from the user.
The app/views subdirectory holds the display templates to fill in with data from our application, convert to HTML, and return to the user's browser.
The app/models subdirectory holds the classes that model and wrap the data stored in our application's database.
The app/helpers subdirectory holds any helper classes used to assist the model, view, and controller classes. This helps to keep the model, view, and controller code small, focused, and uncluttered.