<p>Is the singleton class in Ruby a class in and of itself? Is it the reason why all objects belong to "class?" The concept is <strong>fuzzy</strong>, but I believe it has something to do with why I can define a class method at all (<code>class foo; def foo.bar ...</code>).</p>

<p>What is the singleton class in Ruby?</p>

The most pragmatic/action-oreinted way to think of it (IMHO) is: as an inheritance chain, or method lookup/resolution order.  This picture might help
http://www.klankboomklang.com/2007/11/25/modules-part-i-enter-the-include-class/
This is r 1.9, contrasting builtin and user-defined classes: i'm still digesting this one.
http://d.hatena.ne.jp/sumim/20080111/p1
Also, i htink a confusing use of the term is "Singleton object", which is different concept.  A singleton object comes from  a class which has its constructor/instantiator method overridden so that you can allocate only one of that class.