In RVM-speak, a "Ruby" is a version of Ruby either installed or it knows has to install. Before you can instally and Rubies, you first have to know which Rubies RVM knows about. The command to do this is rvm list known.
Running the rvm list known command will output a list of known Rubies. Among this list are the normal Ruby MRI versions, usually at the top. Further down the list are are other implementations like jRuby, Ruby Enterprise Edition, etc.
$ rvm list known
The format of these names is important. You'll notice that portions of these names are in parentheses. The portions in the parentheses can be omitted when referring to that version on the command line. For example, the Ruby called jruby is really jruby-1.4.0. If you want another version of jRuby, you'll need to use the full name, such as jruby-1.3.1.
To know which Ruby any particular name refers to, you can issue the command rvm info 1.9.1, replacing 1.9.1 with the name of your choice. Among a lot of other information, this command will tell you exactly which version of which Ruby the string refers to.