Hiera is an information repository for Puppet. Using Hiera, you can have a hierarchical categorization of data about your nodes that is maintained outside of your manifests. This is very useful for sharing code and dealing with exceptions that will creep into any Puppet deployment.
At the time of writing, the current release of Hiera is version 3.4.2, and to make things extra confusing, this version includes Hiera5. Hiera configuration versions do not match the release of Hiera itself. Originally Hiera was just known as Hiera, then incompatible settings were introduced when Puppet4 was released. The Hiera configuration that supported Puppet4 was named Hiera4; now with Puppet5, we have Hiera5, but the actual version number of Hiera is still only 3.x.
Hiera5 introduces to Hiera the ability to run custom puppet functions to lookup information. Hiera5 in conjunction with Puppet5 supports the ability to have custom hiera look up data within Puppet modules. We'll see more of...