The traditional definition of Chemistry goes something like this: Chemistry is the study of the nature, properties, and composition of matter, and how these undergo changes. That served as a perfectly adequate definition as late as the 1930s, when natural science (the systematic knowledge of nature) seemed quite clearly divisible into the physical and biological sciences, with the former being comprised of physics, chemistry, geology and astronomy and the latter consisting of botany and zoology. This classification is still used, but the emergence of important important fields to study such as oceanography, paleobotany, meteorology, pharmacy and biochemistry, for example, have made it increasingly clear that the dividing lines between the sciences are no longer at all sharp. Chemistry, for instance, now overlaps so much with geology (thus we have geochemistry), astronomy (astrochemistry), and physics (physical and analytical chemistry) that it is probably impossible to devise a really good modern definition of chemistry, except, perhaps, to fall back on the operational definition: Chemistry is what chemists do!
Chemistry plays an important part in all of the other natural sciences, basic and applied. Plant growth and metabolism, the formation of igneous rocks, the role played by ozone in the atmosphere, the degradation of environmental pollutants, the properties of lunar soil, the medical action of drugs, establishment of forensic evidence: none of these can be understood without the knowledge and perspective provided by chemistry. Indeed, many people study chemistry so that they can apply it to their own particular field of interest. Of course, chemistry itself is the field of interest for many people, too. Many study chemistry not to apply it to another field, but simply to learn more about the physical world and the behaviour of matter from a chemical viewpoint. Some simply like "what chemists do" and so decide to "do it" themselves.
Chemistry is a way of studying matter. What is matter? As is true with many of those words which are really basic to science, matter is hard to define. It is often said that matter is anything which has mass and occupies space. But then what are "mass" and "space"? Although we can define these, the process yields very little insight into what matter is. So let us just say that matter is anything which has real physical existence; in a word matter is just stuff. Iron, air, wool, gold, milk, aspirin, monkeys, rubber, and pizza - these are all matter. Some things which are not matter are heat, cold, colours, dreams hopes, ideas, sunlight, beauty, fear, and x-rays. None of these is "stuff"; none is matter.
