Isomers are molecules that have the same chemical formula but different structures or arrangements of atoms. This means they contain the same number of each type of atom, but the way these atoms are connected or organized can vary. This structural difference can lead to distinct physical and chemical properties. There are two main types of isomers: structural isomers, which differ in the connectivity of their atoms, and stereoisomers, which have the same connectivity but differ in the spatial arrangement of their atoms. Understanding isomers is important in fields like chemistry and pharmacology, as different isomers can have different biological effects.