The problem with biomass, or pellets that you use for heating your central heating is that you do not know what goes into them. and the manufacturers do not have to state this on the label. So basically it could be any of the following wood:
Chemically untreated by-products and residues from the wood processing industry
Whole trees without roots
Chemically untreated byproducts and residues from the wood processing industry
Forest, plantation and other virgin wood
Chemically untreated used wood
These guidelines can be found here http://www.enplus-pellets.eu/
Depending on where in the world your pellets are made will generally dictate what is in your pellet. but it will mainly be softwoods.
With Food grade pellets Smokin Monkey is right, there is no standard as such, however there are 2 main ways that BBQ Pellets are made, from either whole trees or leftover residuals from other manufacturing processes, such as flooring or furniture manufacturing.
When BBQ pellets are made with whole logs, you get a small percentage of bark in the final product. This adds to the flavour complexity of the smoke coming from the pellets. It is more reminiscent of the original BBQ when people cooked using chunks of wood.
The other method of making pellets are not made from whole trees, but rather from the residuals of kiln dried sawdust. This could be cherry from a cabinet maker or oak from a flooring manufacturer. They make a great pellet, as they burn well, but the smoke from this residual sawdust does not possess the same flavour consumers get from a whole log pellet, another issue I believe with using kiln dried sawdust is that you are not totally guaranteed exact quality if using different suppliers for your dust.
Flavoured pellets will have 30-40% of the desire wood and then 60-70% of either Oak or alder as a base wood, (companies do not like to use the word filler, but that is what they are) So your Hickory Flavour will have 40% Hickory & Oak as a filler. There 3 reasons for this (or so I am told):
1) Many species of trees produce too strong a taste for a lot of people, for example, mesquite.
2) All wood burns at various heat output levels. So rather than have to adjust the grill for each species the user wants a consistent heat output pellet.
3) Some woods are very expensive because of availability, for example, Apple or other fruitwoods. Therefore a 100% pellet using apple and the price could be approximately double therefore putting it out of the range of many customers.
Other flavoured pellets use a base wood (depending where the pellets are manufactured) and then add flavoured oils into the mixing process.
Without going into pellet manufacturing 101 It is not as simple as throwing logs in at one end of a machine and pellets come out of the other end as there isn’t one machine that does it all. Depending if you are using logs or kiln dried dust the process is roughly as follows:
You do not necessarily need to know all the above information, but I feel you should be aware of the different types of pellets out there and also know what you are paying for.