Let you change the global Ruby version on a per-user basis.
Provide support for per-project Ruby versions.
Allow you to override the Ruby version with an environment variable.
In contrast with rvm, rbenv does not…
Need to be loaded into your shell. Instead, rbenv's shim approach works by adding a directory to your $PATH.
Override shell commands like cd. That's dangerous and error-prone.
Have a configuration file. There's nothing to configure except which version of Ruby you want to use.
Install Ruby. You can build and install Ruby yourself, or use ruby-build to automate the process.
Manage gemsets. Bundler is a better way to manage application dependencies. If you have projects that are not yet using Bundler you can install the rbenv-gemset plugin.
Require changes to Ruby libraries for compatibility. The simplicity of rbenv means as long as it's in your $PATH, nothing else needs to know about it.
Prompt you with warnings when you switch to a project. Instead of executing arbitrary code, rbenv reads just the version name from each project. There's nothing to "trust."