There are essentially only 4 types of fuel sources used to heat today's smokers. These are charcoal, propane, electric and wood pellets.
I've heard of people using corn-cobs as fuel, but I've never tried it and I'm not sure I would want to. For any smokers out there using other types of fuel, all I can say is "Yuck" and "good luck".
Charcoal
Charcoal barbecue smokers are probably the most popular and most widely available. More styles of smokers make use of charcoal over any other heat source. Charcoal and wood smokers require the most attention to ensure the temperature and air flow stays optimal. Wood can always be used in place of charcoal in any of the charcoal smokers, but it can be really difficult to keep a steady heating temperature.
Propane
Electric
While meat cooked in an electric smoker is good, most experienced smokers don't believe it as good as food prepared in a smoker using combustion.
Wood Pellet
Pellet smokers are the latest generation of smokers and burn pure hardwood sawdust that has been compressed into pellets. These smokers usually have an auger or some other type of feed mechanism that feeds the pellets into a burner or fire pot at a rate sufficient to keep a steady temperature. Pellet smokers are another "set it and forget it" type of smoker, similar to propane and electric smokers, but the benefit here, is that they are actually using wood as the heat source.
Conclusion
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