Well, bundler solves a problem that's related, but tangential to what rubygems does. Rubygems is first and foremost a packaging format for libraries. It handles loading, provides a common format to specify dependencies (gemspec) and a default code layout for libs. It does not to dependency resolving.

So what people used to call "gem hell" was actually "I need to specify all my dependencies and take care of conflicts myself." That's what bundler does. And it uses rubygems to actually retrieve, install and load the gems.