Rails is a web-application framework that includes everything needed to create
database-backed web applications according to the Model-View-Controller (MVC)
pattern.

Understanding the MVC pattern is key to understanding Rails. MVC divides your
application into three layers: Model, View, and Controller, each with a specific
responsibility.

Active Record, Active Model, Action Pack, and Action View can each be used
independently outside Rails. In addition to that, Rails also comes with Action
Mailer, a library to generate and send emails; Action Mailbox, a library to
receive emails within a Rails application; Active Job, a framework for declaring
jobs and making them run on a variety of queuing backends; Action Cable, a
framework to integrate WebSockets with a Rails application; Active Storage, a
library to attach cloud and local files to Rails applications; Action Text, a
library to handle rich text content; and Active Support, a collection of utility
classes and standard library extensions that are useful for Rails, and may also
be used independently outside Rails.

See also: https://rubyonrails.org/