What sits in YOUR_PLUGIN.rb and YOUR_LIB.rb ?

	require 'sketchup.rb'
	class YOUR
	def YOUR::function()
	#here a function definition
	end
	end

view raw YOUR_LIB.rb hosted with ❤ by GitHub

	require 'sketchup.rb'
	class YOUR_PLUGIN
	def initialize()
	#here Your plugin definition
	end
	end#class

view raw YOUR_PLUGIN.rb hosted with ❤ by GitHub

So at last there are two files that in fact holds allfunction definitions.
First YOUR_LIB.rb that holds all function definitions.
All functions are defined in a separate class (YOUR) to avoid crashes with another people's plugins.

YOUR_PLUGIN.rb holds definition of your plugin, what happens when it is initialised, what happens when You move cursor, etc.

In fact You could define all the functions You need in not in YOUR_LIB file but in a YOUR_PLUGIN , but if Tou want to use a function in another plugin, You just have to call it from library of functions , and not copy it in a plugin file.