Isomers are two or more molecules with the same molecular formula but different stuctural arrangement. There are three types of structural isomerism, these are: functional group isomerism, position isomerism and chain isomerism.
Functional group isomerism is when the functional groups differe i.e. they are from a different homologous series.
Position isomerism is when important groups are at different positions on the carbon chain.
Chain isomerism occurs when there is the possibility for a chain to be branched or straight.