Flue piping is what the combustion gases pass through, after they leave the appliance, but before they leave the home. The metal is usually made of galvanized sheet metal. There are two major types; single wall, and B vent. for standard appliances. Single wall is usually used for short runs to the chimney. B vent is used when going  a long distance, for example, through an attic.

Should the hot flue pipes run through a very cold attic, there is the potential for the combustion gases to condense and form an acidic liquid. Should this happen, the acidic liquid could erode through the flue piping. The B vent type of flue piping consists of a pipe within a pipe. This two pipe system acts as an insulator, allowing the gases to stay hot and therefore not condense inside the pipe.

There is another material that flue pipe can be made from. That is stainless steel. This is typically used for more modern appliances because the modern appliances run much hotter.  The hotter flue gas temperatures makes it less likely that the flue gases will condense. There is less acidic water to be drained out.   The piping needs to be able to handle this increase in extreme heat and stainless steel does a good job with out breaking down.