What are the magic $-prefixed variables in Ruby?

They are called "global variables" (complete list at the bottom of the page): http://www.rubyist.net/~slagell/ruby/globalvars.html

The Ruby documentation used to be very class orientated. In recent versions of Ruby however there are rdoc files about literals, precedence, syntax, globals and much more.

Their name is global variables. There are several different references.

You can get a full list by calling the method Kernel#global_variables

puts global_variables

Ruby also includes a file called "English.rb" in the standard library which provides an in-depth explanation of several global variables.

Also, there's (an archived version of) "Cryptic Ruby Global Variables and Their Meanings".

Finally, the Ruby Programming wikibook has a "Predefined Variables" reference.