Look at the cutaway drawings for the stoker stoves and the handfired stoves on this site: http://www.harmanstoves.com
A stoker stove takes a small amount of coal from a hopper, pushes this onto a grate where the coal is burn as hot as it can burn..The heat output is regulated by how much coal is pushed onto the grate therby regulating the fire size.
A hand fired stove has a large quantity of coal in a firebox, the entire quantity of coal is exposed to the fire from the bottom up, the heat output is controled by regulating the air to the fire..the area of coal is fixed, so the heat output is not as controlable as with a stoker stove. As the coal burns, the depth of the fire gets shallower, and as the coal burns out, the fire burns out when the coal depth becomes too shallow to support a fire
A hopper fed stove drops fresh coal into the firebox as the batch of coal burns down, the hopper keeps the depth of the fire constant for many hours longer, sometimes up to a day longer than a simple hand fed stove.. This prolongs the time between loading to keep the fire active.