There are a few types of bearings used in bicycles. Two of these are cup-and-cone and cartridge bearings.
@Ajax pointed this out in his post above.
Note that both these bearings can be sealed or unsealed. Further, there are different types of seals, each with their own merits.
In spite of people's perceptions that cup-and-cone bearings are inferior to cartridge bearings ("sealed" in the wrong parlance), this is not the case. Cup-and-cone bearings are angular contact bearings which can work with both radial and axial forces, whereas cartridge bearings of the deep groove ball type, can only work with radial forces. The approximately 45 degree contact angle of the latter copes better with bicycle-generated forces on wheels and BBs than cartridge bearings. They are also more expensive to manufacture and harder to service, although, I'll dispute the latter.
Cartridge bearings are favoured by manufacturers because they are off-she-shelf items that don't require any design skills or special tools at bike manufacturer level.
Lastly, the diagram offered upstream, from the Sheldon Brown website, has errors and omissions in the labelling. The "dustcap" is actually a contactless labyrinth seal and the labels indicating the two contact rubber seals on the left and right side, are missing.