a package management framework for the Ruby programming language
An application or library is packaged into a gem, which is 
a single installation unit. 
RubyGems entirely manages its own filesystem space, rather 
than installing files into the "usual" places. This enables 
greater functionality and reliability. 

Using RubyGems, you can: 
- download and install Ruby libraries easily 
- not worry about libraries A and B depending on 
  different versions of library C 
- easily remove libraries you no longer use 
- have power and control over your Ruby platform! 

WWW: http://rubygems.rubyforge.org/wiki/wiki.pl