Hempy is a hydroponic system that uses buckets typically requiring hand-watering. The name Hempy came from the creator of the system "Hempy".
Simply put, you take any size buckets (I'm doing a hempy system right now with a series of 2L Coke bottles), drill a 3/8 inch hole about 2" from the bottom of the bucket for draining, and the bottom of the bucket is the actual build-in res. Every day or couple of days, you top up the bucket with fresh feed.
Usually, you start off watering daily until your root system reaches down into the build-in resevoir. The original hempy consisted of a mix of perlite and vermiculite, but nowadays people are doing all sorts of mediums in them. Personally, I prefer 100% hydroton as it is very easy to reuse after each grow, but I also have a few with 100% perlite.
Does that help you understand better?
-spek