Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds. For example: Butane. There are endless possible ways that the molecule could twist itself. There is completely free rotation around all the carbon-carbon single bonds. If you had a model of a molecule in front of you, you would have to take it to pieces and rebuild it if you wanted to make an isomer of that molecule. If you can make an apparently different molecule just by rotating single bonds, it's not different - it's still the same molecule.

In structural isomerism, the atoms are arranged in a completely different order. The different types of structural isomerism are:

• Chain isomerism: It arises because of the possibility of branching in carbon chains. Example: C4H10. One is branched and other is straight chain.
• Position isomerism: In position isomerism, the basic carbon skeleton remains unchanged, but important groups are moved around that structure like halogen, acid, etc group. Example: C3H7Br
• Functional group isomerism: In structural isomerism, the isomers contain different functional groups - that is, they belong to different families of compounds (different homologous series). Example: C3H6O  could be either propanal (an aldehyde) or propanone (a ketone).