RubyGems is a way to package Ruby libraries/applications for
distribution, its a distribution manager, its a package manager.
RubyGems brings to the world of Ruby a very valuable capability--the
ability to manage concurrent versions of libraries and dependencies
between those libraries.  This last aspect of RubyGems is one of the
most important to the end user, because its value is continuous as you
use and program Ruby, but to understand its impact an explanation of
how Ruby manages libraries is in order.