a.rb

puts "Start parsing A"

require_relative 'b'

puts "Finish parsing A"

print "$foo_global_variable: "
p $foo_global_variable

print "@@foo_class_variable: "
p @@foo_class_variable

print "@foo_instance_variable: "
p @foo_instance_variable

print "foo_local_variable: "
p foo_local_variable

b.rb

puts "Start parsing B"

$foo_global_variable   = :bar
@@foo_class_variable   = :bar
@foo_instance_variable = :bar
foo_local_variable     = :bar

puts "Finish parsing B"

output.log

Start parsing A
Start parsing B
/Users/Justin/Dropbox/Code/playground/b.rb:4: warning: class variable access from toplevel
Finish parsing B
Finish parsing A
$foo_global_variable: :bar
a.rb:11: warning: class variable access from toplevel
@@foo_class_variable: :bar
@foo_instance_variable: :bar
foo_local_variable: 
a.rb:17:in `<main>': undefined local variable or method `foo_local_variable' for main:Object (NameError)