For my own sake, I want to write this down while I still remember it.

class_eval executes a block in the context of a Class. instance_eval executes a block in the context of an instance. When you do def inside a class_eval, you always define a new instance method. (Unless you do def self.something in which case you define a Class method, same as always.)

When you do def inside an instance_eval, it depends on what exactly your receiver is. If you do a def inside an obj.instance_eval, you define a singleton method for obj, which affects no other objects of the same class. If you do a def inside a SomeClass.instance_eval, you define a class method for that Class.

So these all seem to do the same thing, and allow you to do Foo.foo<.

# ONE
class Foo
end

def Foo.foo
  puts "FOO"
end

# TWO
class Foo
  def self.foo
    puts "FOO"
  end
end

# THREE
class Foo
end

class << Foo
  def foo   # corrected, thanks Rupert
    puts "FOO"
  end
end

# FOUR
class Foo
  class << self
    def foo
      puts "FOO"
    end
  end
end

# FIVE
class Foo
end

Foo.class_eval do
  def self.foo
    puts "FOO"
  end
end

# SIX
class Foo
end

Foo.instance_eval do
  def foo
    puts "FOO"
  end
end