Stoker refers to a coal boiler or hot air furnace that has a mechanism to automatically feed and fire coal. There is two common types, underfed pots with an auger and bed designs with a pusher block. Both provide forced combustion air through holes or gaps located in the pot or bed.
The big benefit of a stoker is automation, they operate very much like any conventional boiler that is thermostatically controlled. The big difference of course is you need to supply coal to it and remove the ashes.
For residential heating the BTU output typically ranges from 70,000 BTU to 250,000 BTU but can go higher.