Substance - the actual matter of a thing, as opposed to the appearance or shadow; reality.
Leaning down to greet a new client; a German Shepherd puppy I grab hold of her front leg, nice substance. I like a dog with substance, beef, good bone, meaty. Sadly many of our breeds are taking a dive in the substance department. Many breeders are heading down the path the leads to injury and structural problems as they adjust their goal to the "refined" look. A longer neck, a lighter gate or refined face.
Luke does not have substance; he is from show lines and has a wonderful long neck, beautiful gate but he is always hurting himself. When I grab his front leg I shutter at how little substance there is compared to Tilley or Elsa. I want to feel a handful when I grab a leg; something that tells you that this dog could hold up after a long day of work.
Structure - a complex system considered from the point of view of the whole rather than of any single part.
But substance does not stand alone; you can have substance without structure or balance and a dog will suffer. I am not talking about what is required in a show ring; I am talking about work-ability, movement, ease in day to day life and being able to play hard and not suffer for it. Some breeds have gone the other direction with so much substance that they can barely move. This is as much as a hindrance as having too little substance.
Take the German Shepherd for example; a fabulous dog that has undergone extreme change over the years. The dogs seen in the show ring are no where near the dogs of years gone by. They have lost so much in the rear that many cannot lead a normal day to day, let alone a working life. Why has this happened? Humans are a weird bunch, if a tiny slope is good then an extreme slope would be better correct? No, not even close.
How about the Dachshund? Again another great dog but structure? Really? It is obvious when you look at the breed that they have back issues, how could they not? Substance? Yes many have great substance but every single one lacks in the structure department. They may be the best show specimen ever but are they a structurally sound dog? No.
When you follow along the years and watch a breed transformation it can be a bit scary to say the least. Depending on a breeders eye and what is in at the moment can have a drastic impact on results. Many of the "big hair" breeds suffer due to the focus put on hair instead of what's under the hair. All dogs are meant to run and play and maybe work; but with human intervention much of that has now changed. I see far too many breakable dogs these days, it is a sad truth.