What is a class?

A text-book answer: classes are a blue-print for constructing computer models for real or virtual objects... boring.
In reality: classes hold data, have methods that interact with that data, and are used to instantiate objects.
Like this.

class WhatAreClasses
  def initialize
    @data = "I''m instance data of this object. Hello."
  end

  def method
    puts @data.gsub("instance", "altered")
  end
end

object = WhatAreClasses.new
object.method
 #=> I''m altered data of this object. Hello.