There are various types of wood used to smoke your food which will mainly depend on the device you are using.
Wood chips are the most common and are used with larger smokers e.g. ProQ Frontier, ProQ Excel, Brinkmann Smoke n Grill etc. It is best to use them in a smoker box
which allows the wood chips to smolder and therefore produces a constant stream of rich smoke. Can also be used on a gas BBQ with a domed lid.
Wood chunks are with larger smokers e.g. ProQ Frontier and Brinkmann smokers and are usually placed directly on the fire. They can be soaked in water before use which will allow them to burn for longer.
Please note using treated wood or wood that you are not able to establish its origin can be a health hazzard. Treated wood can give off toxic fumes and should be avoided. Some woods like willow and pine are in themselves toxic and should not be used for BBQ food smoking.