Bulk carriers (bulkers) carry dry bulk cargoes: i.e. cargoes that can be handled by conveyor systems, chutes, grabs, or skips, e.g. minerals, grain, fertiliser, bulk sugar.
Some bulkers carry neo-bulk cargoes: i.e. cargoes that are made up of a large number of individual pieces, but that can fill holds. e.g. pig iron, bagged cargoes, steel, aluminium ingots, timber, rolls of newsprint, blocks of granite, and many other cargoes like this.
Some are gearless (they have no cranes).
Some are geared (they have cranes).
Some are larger and/or specially designed to carry large volumes of coal or iron ore.
Some are smaller to allow them to enter smaller, shallower ports, and/or to carry smaller parcels of cargo.