For a longer list (though the explanations are briefer), see Zenspider's Ruby QuickRef[1]. It also includes a handy listing of predefined global constants[2] (and a lot more).[3]
[1]: http://zenspider.com/Languages/Ruby/QuickRef.html#pre-define...
[2]: http://zenspider.com/Languages/Ruby/QuickRef.html#pre-define...