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

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.