Many chemical compounds, especially hydrocarbons, can exist in different geometric configurations. A structural formula represents the arrangements of atoms in a way that a chemical formula cannot.
A simple example of this may be seen with the hydrocarbonbutane, C4H10. The four carbons may be arranged in a linear pattern, or in a branched, "T" pattern. The first arrangement is known as orthobutane, while the second is isobutane.