Isomerism: The compounds having the same molecular formula but different structural formula are referred to as isomers and phenomenon as isomerism.
Examples :
${\mathrm{C}}_4{\mathrm{H}}_{10}-\mathit{n}$ -Butane
${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3$ - Isobutane
${\mathrm{CH}}_3-\underset{{\mathrm{CH}}_3}{\underset{|}{\mathrm{CH}}}-{\mathrm{CH}}_3$