A stereoisomer is a type of isomer that has the same molecular formula and connectivity of atoms but differs in the three-dimensional arrangement of those atoms in space. This spatial arrangement can lead to different chemical properties and behaviors, even though the basic structure remains the same. Stereoisomers can be further classified into two main categories: geometric isomers and optical isomers. Geometric isomers occur due to restricted rotation around a bond, often seen in compounds with double bonds or rings. Optical isomers, on the other hand, are molecules that are non-superimposable mirror images of each other, much like left and right hands. These differences can significantly impact how substances interact with biological systems, making stereoisomerism an important concept in fields like pharmaceuticals and biochemistry.