This information exists here and there, but I thought I'd try to capture simply in one place.
If you are running ubuntu linux and you want to install ruby the you currently have to install several packages. For some reason, some of those packages are on the approved ubunto list (e.g. ruby) and some (e.g. irb) are not. I suppose because some people will need to run a ruby program but not need the interactive interpreter.
To get to the "unapproved" packages for ruby you need to edit /etc/apt/sources.list
There is a comment in the file about getting access to the "unapproved" universe. Uncomment the denoted lines of the file. What's not immediately clear though is you have to run "apt-get update" before the next "apt-get install ..." will recognize the newly available sets of packages.
After the update then do the following installs... (Your list will vary if you want rails, etc. You probably want to leave out the 1.8, that's just the current version.)
apt-get install ruby1.8 apt-get install ri1.8 apt-get install rdoc1.8 apt-get install irb1.8 apt-get install ruby1.8-elisp apt-get install ruby1.8-examples apt-get install libdbm-ruby1.8 apt-get install libgdbm-ruby1.8 apt-get install libtcltk-ruby1.8 apt-get install libopenssl-ruby1.8 apt-get install libreadline-ruby1.8
Apparently rubygems has to be installed the old fashioned way.