title: Ruby on Rails inheritance syntax
body: <p>Given this piece of Code:</p>
  
  <pre><code> class CreateComments &lt; ActiveRecord::Migration
    def change
      create_table :comments do |t|
        t.string :commenter
        t.text :body
        t.references :article, index: true
  
        t.timestamps
      end
    end
  end
  </code></pre>
  
  <p>I understand that CreateComments inherits methods from Migration. But what is Migration to ActiveRecord? Compared to other languages, I could not find any member named Migration in the ActiveRecord class. So what does the <code>::</code> mean? What do you call Migration in this case?</p>