What are "structural isomers"?

Organic chemistry provides rich opportunity for structural isomerism. Even for a simple hydrocarbon, say $\text{butane} , {C}_{4} {H}_{10}$, two structural isomers may be drawn which have the same formula but different connectivities: viz. $\text{n-butane} , {H}_{3} C - C {H}_{2} C {H}_{2} C {H}_{3}$, and $\text{isobutane, } {H}_{3} C - C H \left(C {H}_{3}\right) C {H}_{3}$. These structural isomers have different physical and chemical properties, i.e. boiling points, $- 1$ ""^@C, versus $- 11.7$ ""^@C respectively.
As more carbons are added to the formula, the opportunity for structural isomerism becomes even greater. The $\text{pentane}$ formula, ${C}_{5} {H}_{12}$, can generate 3 structural isomers, each with distinct connectivity. For $\text{hexane} , {C}_{6} {H}_{14}$, there are 5 structural isomers. Isomerism is one reason why teachers go to such lengths to teach systematic ways to name organic chemicals unambiguously.
Structural isomerism leads on to the topic of geometric isomerism, where stuctures are the same, i.e. $C - C$ connectivity, but geometry is distinct.