A tracking dog uses its nose to follow a track. For a human it is hard to imagine, what its nose is able to recognize. To give you an idea here are some facts about noses.
A human has about 5 million receptor cell to smell. A ridiculous small amount compared to a dog. A terrier possesses about 147 million and a shepherd dog already has a lot more. 225 million of those cells are located in his nose. But even this is toped by the hounds, who are equipped with more then 300 million receptor-cells.
But the receptor-cells are not the only reason, why a dog is so outclassed. Dogs have something, that does not exist in the human body. It is an organ called the Jacobson´s organ. With this organ a dog is able to literally taste a smell.
Moreover a dog uses a different way to smell. To smell a dog interrupts breathing and draws the air into his nose and to the Jacobson´s organ. Doing this, the dog is able to analyze the scent for its origin, age and compounding. As the result of more then 300 million receptor-cells and the Jacobson´s organ a dog smell 10,000 times as good as a human.
We still do not completely understand, how a dog works with all the information he can discern. We assume, a dog composes a kind of picture in his brain consisting of what he smells, hears and sees.