Chris, just some more thoughts on entities..
entities, in your example, is a variable which points to Sketchup.active_model.active_entities. It is not a copy of Sketchup.active_model.active_entities, but think of it as a shortcut (aka pointer, or reference) to Sketchup.active_model.active_entities.
If you want to copy the current Sketchup.active_model.active_entities, you can call .to_a also:
ents_copy = Sketchup.active_model.active_entities.to_a
Sketchup::Entities is single object, but it is a container of Sketchup::Entity objects. If you would like to know what is a Sketchup::Entity, you can see my API graph.
The Ruby module Enumerable is mixed-in to Sketchup::Entities, which means Sketchup::Enities will respond to the mothods defined in Enumerable.