Rails is made up of several components that all have specific jobs in making your Rails application work. Here’s a quick list of what they are and what they do.

ActionMailer - The basis for mailers. This is the library that allows you to format and send emails from your rails application

ActionPack - The components that parse, delegate, and render responses for requests.

ActionDispatch - Parses HTTP requests and sends the requests to controllers.

ActionController - Basis for Controllers -Takes requests and request parameters and gathers data to render a response.

ActionView - Basis for Views. Determines which template to use and builds the response to the HTTP request from the templates based on data provided by the Controller.

ActiveModel - Defines an interface on non-ActiveRecord classes that helps the behave like ActiveRecord models.

ActiveRecord - Basis for Models. Manages the connection between the Rails application and a SQL database. Provides means of querying database to manage and find records.

ActiveResource - Allows the developer to access network REST resources in a simple way.

ActiveSupport - A collection of extensions to the Ruby programming language that can be used in your Ruby on Rails application.

You can read in more detail what these libraries are about and what they do on the Ruby on Rails github page.