This may be simple, but it allows to start a conversation between two strangers involved into the same craft.
Classes are a blue-print, they may hold data, likely they hold methods; classes are directly connected with an idea of objects, because object is an instance of a class. As objects are first-class citizens in Ruby, there is a main, root class Object, and all classes are inherited from this root entity. Modules, generally, are a tool for mix-ins and they provide something we call a namespace. Modules cannot be initialised the way we can do this with classes, but they provide a multiple inheritance.