Conduction, Convection and Radiation

Heat always flows spontaneously from hot to cold, never the reverse. But it can take three different routes, which you need to be able to tell apart.

   mode           mechanism                        example          medium?
   ------------   ------------------------------   --------------   ------------
   conduction     step by step, without            spoon in         solid
                  any movement of matter           coffee
   convection     by movement of a hot fluid       radiator that    liquid/gas
                  (which rises)                    heats a room
   radiation      by electromagnetic waves         heat from the    none
                  (infrared)                       Sun, from fire    (even vacuum)

Conduction passes thermal agitation from atom to atom, without the matter moving: a spoon standing in a hot drink becomes hot all the way to the handle. Metals excel at it; air and wood are poor conductors — and therefore good insulators.

Convection only exists in fluids: a heated fluid expands, becomes less dense and rises, while the cold fluid sinks. This looping motion carries heat much faster than conduction. This is how a radiator heats a whole room, and how water boils in a saucepan.

Radiation is the only one that requires no medium: the Sun's heat reaches us across the vacuum of space, in the form of electromagnetic waves (mostly infrared). Every hot body radiates.

   conduction  : contact, no movement of matter
   convection  : movement of a fluid
   radiation   : EM waves, crosses a vacuum

In reality, all three often coexist. A fireplace fire heats by radiation (what you feel on your face) and by convection (the hot air rising), while the handle of the poker heats up by conduction.