An isomer is a molecule with the same molecular formula as another molecule, but with a different arrangement of atoms.
Structural isomers occur when "two or more organic compounds have the same molecular formulae, but different structures," according to Creative Chemistry. That means these differences can give the molecules different physical and/or chemical properties. There are three main types of structural isomerism: chain isomerism, functional isomerism and positional isomerism.
Stereo isomers occur when "the atoms in a molecule can have different arrangements in space." There are two main types of stereo isomers: optical isomers and geometrical isomers. Geometrical isomers can have some different physical properties, like different melting points, but they usually have the same chemical properties. Optical isomers have both the same chemical and physical properties, according to Creative Chemistry, "except that one structure rotates the plane of polarised light to the right and the other rotates it to the left."