Stereoisomerism is a form of isomerism where molecules have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their atoms. This results in different properties and behaviors, even though the chemical composition is identical. 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 significance of stereoisomerism is particularly evident in biochemistry, where the different spatial arrangements of molecules can lead to varying biological activities. For example, the two optical isomers of a drug may have different effects in the body, making stereoisomerism crucial in the development of pharmaceuticals.