Isomers are molecules that share the same molecular formula but have different structures. However, the structural difference does not include the rotation of the molecule or rotation around particular atomic bonds. Rotation of the molecule around its bonds or as a unit does not constitute a strong structural difference.
Chain isomers are molecules formed when in one isomer the atoms are arranged in a straight line or when in another the atoms are branched.
Position isomers are isomers formed when the location of the “special” atom is changed and bonded in different positions along the base structure.
Functional group isomers are isomers that are formed when different functional groups are developed by altering the structure of the molecule. Thus, the molecule maintains the same formula, but the isomers belong to different compound groups. For instance, propanone and propanal.
Find a visual representation of the structural differences here Chemguide.