Chemistry is the science that tells us what things and people are made of; for example, that water is really H2O, two atoms of hydrogen and one of oxygen.
Chemistry deals with chemicals and elements, the building blocks of our world. The periodic table, that table of all of earth's basic elements — iron, carbon, oxygen, nitrogen, etc. — comes from chemistry. Chemistry explains how atoms attract each other to become molecules. We also use the term to refer to people who are attracted to each other, either as colleagues, friends or romantic partners. Movie reviewers often say that that romantic leads have "great chemistry" — or not.