Differences_between_the_evals.rb

klass = String
metaclass = class << klass; self; end
baz = "baz"

metaclass.class_eval(%\def foo; 'foo'; end\)
# defines class methods
# klass.foo = 'foo'
# String.foo = 'foo'

metaclass.instance_eval(%\def foo; 'foo'; end\)
# creates methods for that metaclass instance
# String.foo = error
# metaclass.foo = 'foo'
# klass.foo = error   

klass.class_eval(%\def foo; 'foo'; end\)
# creates an instance method
# "bar".foo = 'foo'

klass.instance_eval(%\def foo; 'foo'; end\)
# creates class methods
# klass.foo = 'foo'
# String.foo = 'foo'

String.class_eval(%\def foo; 'foo'; end\)
# creates instance methods
# "bar".foo = 'foo'

String.instance_eval(%\def foo; 'foo'; end\)
# creates class methods
# String.foo = 'foo'
# klass.foo = 'foo'

baz.instance_eval(%\def foo; 'foo'; end\)
# creates instance methods for that particular instance
# baz.foo = 'foo'
# "bar".foo = error