In JavaScript they're the same thing - objects do quintuple-duty as hashtables, records, objects, classes, and modules. Arc is very similar to JavaScript in terms of the primitive data types available - they're both Scheme-derived languages.
The reason I'd term call this modules is that they don't have the implicit-receiver that classes/objects do, nor do they have constructors. Once you define a module, that's it, and they live in the global namespace. You could change things around pretty easily so that instead of immediately executing the closure containing the module's namespace, you define it as a constructor function. And you could probably then come up with some macro to expand to a message send, which passes the object as the first argument to any method defined in it. I think you'd be stuck with explicit-self though, like in JavaScript or Python, instead of the implicit `this` of Java.
