Many ships have large spaces below their decks to hold cargo. Bulk carriers are built to carry dry cargo and tankers to carry liquids.
The holds of bulk carriers that carry dry cargo have large openings called hatches. The cargo holds are filled with large volumes of dry materials like wheat, woodchips, coal, iron ore or other minerals.
Oil tankers have holds that are large tanks - these do not have hatches. Liquid such as oil is piped into the tanks. The tanks are sealed so that the contents can't escape and pollute the environment.
Chemical tankers have smaller specially constructed tanks which carry a wide range of chemicals in bulk such as sulphuric acid, caustics, vegetable oils etc.