Commodity trains are where the train hauls one specific type of load. That could be anything from coal, to ore, to sand. The type of car is generally the same, but does not have to be. Creating commodity trains is real easy. You simply have to make sure that this train hauls the commodity, but no other. That is done in the edit layout section, under the Trains tab, Train's Commodity sub-tab. Then you have to make sure you have cars that can haul that commodity. If the cars are strictly for that commodity and none other, then those cars will be on that train only. An ore train or log train may be an example of that as it is unlikely that the ore cars would be used on other trains (with exceptions below).
You may have to make a special car type to facilitate that if the same car type is used for other commodities, especially if you want the same cars to be on the commodity train over and over. For example, a coal train may use 3-bay hoppers, but other commodities and trains also use 3-bay hoppers. You may want to have a specific group of these hoppers only be on the coal train. Hence make a car type 3-bay hopper coal, and assign those cars to that type.
There may also be variations on how you ship commodities on the layout such that a commodity train brings up one specific commodity, but other trains deliver that commodity as part of their consist. The demo program has that with petroleum. There is a train that comes up the line from Lambton that is a petroleum commodity train. The branch line trains then haul those tank cars to their respective industries.
This is done by making sure that only the commodity train and the branch trains, can haul that commodity. Simple.