Constitutional (structural) isomers are compounds that have the same molecular formula (same constitution) but a different atom-to-atom bonding sequence. Since constitutional isomers have different physical properties, usually they can be separated easily from each other.
Constitutional isomers of C4H9Cl are used below to illustrate different atom-to-atom bonding sequences.
(1) CH3CH2CH2CH2Cl
(2)
(2)
Constitutional (structural) isomers may have the same functional group and the same carbon skeleton (The carbon skeleton shows the arrangement of carbon atoms.), but the functional group is at a different carbon atom.
Examples of this type are the constitutional isomers (1) and (2).
This type of constitutional isomerism is also known as positional isomerism.
This type of constitutional isomerism is also known as positional isomerism.
(3) 
Constitutional (structural) isomers may have the same functional group and different carbon skeletons.
Constitutional isomer (3) is a C4H9Cl isomer with a carbon skeleton different from that of (1) and (2).
Constitutional isomers that have different carbon skeletons are also known as chain isomers.
Constitutional isomers that have different carbon skeletons are also known as chain isomers.