PS hairs are pre-sim hairs.  These are hairs that are generated before the simulation, then simulate along with the guide hairs.  

PR hairs are pre- render hairs.  They are generated at render, and show up around the simulated hairs.

The generate option should just be left alone.  If you shut those off, you will be left with guide hairs only. Great if you want to export these out to another program.

The preview PR hairs option allows you to see the pre-render hairs in the viewport ( if the vendor used any PR hairs to help fill it in).  If your viewport is struggling, you can shut that off and use it when you need to.