Stereoisomers are molecules that have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of those atoms. This difference in arrangement can lead to distinct physical and chemical properties, even though the molecules are composed of the same elements. Stereoisomers can be classified into two main types: geometric isomers, which differ in the arrangement around a double bond, and optical isomers, which are non-superimposable mirror images of each other. The study of stereoisomers is important in fields like organic chemistry and pharmaceuticals, as the different arrangements can affect how a substance interacts with biological systems. For example, one optical isomer of a drug may be effective, while its mirror image could be inactive or even harmful. Understanding stereoisomerism helps chemists design better drugs and predict their behavior in the body.