A hopback allows you to hop your brew just before cooling it. The advantages here are, of course, the ability to add hop flavor, without adding bitterness. This is a somewhat similar result to dry-hopping.
I decided to build a hopback when I bought a plate chiller. I was dissatisfied with my immersion chiller, which I felt took to long to cool the wort. So, I bought a plate chiller, which is a smaller version of what the big breweries use. Basically, you have cold water running between copper plates counterflow to your wort.
When I was doing some research about plate chillers, I found that it was good to put a hopback in the line before them, after your brew kettle. This will keep the hop solids and hot break from clogging the plate chiller.
I found a couple of websites explaining how to make a hop back, so I read them , got some ideas, and then made my own. Here are my notes on how I made mine.