A structural formula is a
diagram of a
molecule, showing which
atoms are
bonded, and how they
bonded together.
An example of a structural formula:
H H
\ /
C -- C
// \\
H -- C C -- H
\ /
C == C
/ \
H H
benzene
The single lines are regular
single bonds, while double-lines are
double bonds. You will occasionally also come across
triple bonds and
hydrogen bonds, though hydrogen bonds are typically only displayed as a regular single bond.
Structural formulas don't necessarily show the actual shape of the molecule, especially since the molecules exist in 3D space and these diagrams are only 2D representations. Their purpose is more focused on showing what is bonded to what, and by what kind of bond.