Pawn is an in-game addon that helps you to evaluate gear to determine which piece is better. The addon allows you to assign points to different stats and then tells you how many points a certain piece of gear is worth according to your point scale. In theory, if you have two pieces of gear, the one with the most “points” is superior. In reality, Pawn is only as good as your scales.
In the default UI you choose stats and assign point values to them. You can also import fully-formed pawn scales into the system.
After the scale is setup pawn will add the “points” to the tooltip. Here we see my pseudo points for a few pieces of gear on my warlock. According to my gear scale, the Gown (which I’m wearing) is superior to the robes for DPS. That said, the robes have a nice chunk of +hit on them so I keep them around for swapping gear. The numbers in parenthisis are the “raw” gear score. The number outside the parentisis is the score of this gear including gems and enchants.
As mentioned, the gear suggestions are only as good as the scale. To remain up-to-date you must adjust the scale as patches come out and classes are changed to value some stats over others.
There’s an unofficial Pawn forum where you can find scales or I’ve seen them published on Elitist Jerks.