Prometheus is a prototype-based message-passing object system. This means that there are no classes, but instead, objects are created and modified on the fly until they match the specified behavior. Then, these objects can be "cloned" into a new object which inherits the whole behavior of the parent objects. An object in this world is just a set of named slots which can be accessed or run by sending a message to the object.
In Prometheus, objects are closures that receive as the first argument a message selector, and arguments to the message as remaining arguments. See the Prometheus manual for further information.