Modules are collections of methods and constants. They cannot generate instances. Classes may generate instances (objects), and have per-instance state (instance variables).
Modules may be mixed in to classes and other modules. The mixed-in module’s constants and methods blend into that class’s own, augmenting the class’s functionality. Classes, however, cannot be mixed in to anything.
A class may inherit from another class, but not from a module.
A module may not inherit from anything.