The single wall flue is a simple stainless steel duct. When hot gases move through the non-insulated flue, its outer surface becomes extremely hot. This is not ideal if the flue needs to run near combustible materials such as wood trusses or ceiling boards etc, however they are good for providing additional heat to the room.
Insulated, double wall flues are made of two single wall flues with 50mm difference in diameter. The space between the two ducts is filled with insulating rockwool material. This prevents the outer surface of the flue from getting hot, thus making these flues more suitable for installation near combustible materials. The insulation also keeps the flue gases hotter for longer, improving the draw of your fireplace.
Typically we would use single wall flue piping from your fireplace to the ceiling, and then use double wall flue piping in the ceiling, where there are combustible materials that can easily catch alight from the heat of a single-wall flue.