Direct heat smokers are designed to be used with both charcoal and wood.  In fact, the wood is were your smoke flavor comes from.  So, no they are not charcoal only.  A high quality lump charcoal is preferred.  A winning pitmaster method is to toss 1 or 2 medium sized wood chunks in with your lump.  The type of wood you use all depends on the type of meat you are cooking and the flavor profile you are going for.  *Pro Tips:  Be careful not to use too much wood as this can “over smoke” the meat you are cooking.  1 or 2 medium sized chunks is good for the size of the charcoal baskets our drums are sold with.  However, if your drum is loaded down with a specific meat item you can cook a lot of, like ribs for example, then 3 or 4 chunks might be okay.  Read more on charcoal powered direct heat smokers