In the earlier days of the shipping industry bulk cargo was transported in ships having large cargo holds and these ships were known as bulk carriers. This is a satisfactory method for transporting large quantities of cargo such as say ore, grains etc but sometimes dry cargo needs to be segregated into smaller units each having its distinct owner and local destination.
Just imagine a hundred businessmen ordering let us say cloths from another country. Each businessman has his or her specific order and type of cloths. It would not be very easy to dump all this cargo into one cargo hold and segregate it at the receiving end. A better choice would be to 
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This concept is what is used in container shipping and all the containers are of standard sizes and stored in an orderly fashion on the ship. Just take a look at the picture of a container ship and you will have an idea of how the container ship is designed. You will notice that unlike bulk carriers and tanker ships which carry the material inside their hatches (sometimes they can carry cargo on top as well like log carriers), the container ships carry cargo above the deck level as well apart from the containers in the hatches which may or may not be covered with hatch covers.
Another feature of these ships is that the small sized container ships usually have their own arrangement for loading/unloading of containers while the bigger ones normally depend on the shore facilities for this purpose.
