Wood and wood-derived biomass fuel like wood pellets need the combustion air to come from above to burn effectively, whereas mineral fuels like (smokeless) coal and peat need combustion air to come from underneath.
A Woodburning Stove is only able to burn wood, and either has a small grate or no grate at all. Multi Fuel Stoves have a grate that keeps the fuel up off the bottom and have a riddling system to shake the grate and dump ash into an ash pan. Some Multi Fuel Stoves are also very effective at burning wood and wood-derived biomass fuels. These Stoves have a riddling system that closes the grate, creating a flat bottom.