An isomer in chemistry is a chemical that has the same molecular formula as another, but it has a different arrangement of atoms in space. Different isomers have different chemical properties (that is, they may look, or smell, or taste different from each other).
There are two main categories: stereoisomers and structural isomers. Structural isomers are isomers with the atoms making up the molecule joined together in different ways.
Stereoisomers have different positions of atoms in space and are much more subtle than structural isomers. See enantiomer for stereoisomers that are related to each other by a reflection: they are mirror images of each other that are non-superimposable.