[jffry]

In this case for heating, efficiency is how much of the energy input gets converted into heat in the space you're heating.

So for a resistive electric heater, you're dissipating all of the energy as heat (minor quibbles about electromagnetic radiation or status LEDs aside). The same is true for your computer.

Burning hydrocarbons have two main sources of energy loss: incomplete combustion, and energy carried away by exhaust gases. Obviously furnace design influences both of these - the US federal minimum is 78%, but high-end furnaces with all the tricks can get over 90% per https://www.energy.gov/energysaver/furnaces-and-boilers

Heat pumps are the third major category, and they are more than 100% efficient because they are using their energy to steal a larger amount of thermal energy from the outdoors air and move it to inside your home (and much of the electric energy they consume is eventually discharged as heat inside your home as well).