The concept of isomers in chemistry is very important.  Here is a simple definiton of an isomer:

Isomers - Molecules with the same Molecular Formula but different Structural Formula.   n-butane and iso-butane have different structural formulas, but as we see in the following table both molecules have the same molecular formula.





Both n-butane and iso-butane have the exact same Molecular Formula.  Both have 4 carbon atoms and 10 hydrogen atoms in the molecule, but look again at Figure 9.3 in your textbook.  Notice how the 2 molecules DO NOT have the same Structural Formula. These 2 compounds are different substances.  They have DIFFERENT chemical and physical properties.  They are unique compounds even though they have the same number of carbons and hydrogens in the molecule.  The Structure of the molecule is extremely important in determining what the substance is.  n-butane and iso-butane are Isomers.