i'm thinking could it be single bonds and double bonds?
If u A lvl,
then compare ketone with aldehyde <-- same formula but different compound.
If u O lvl,
I think u should learn something called isomers?
"Two molecules that have the same chemical formula, and have the atoms connected together in the same way but still be different" refer to stereoisomers, rather than structural isomers.
(Note however, the question "still be different" is vague. Depending on other factors, two molecules in different microenvironments, different temperatures, different pressures, different inter-particulate forces or energies acting on them, will naturally have different energy levels and hence "still be different" even if they are actually identical and not stereoisomers.)
Isomers are divided into structural aka constitutional isomers, and stereoisomers.
Structural isomers are subdivided into chain isomers, positional isomers, and functional group isomers.
Stereoisomers are subdivided into geometric (Cis-Trans or E-Z) isomers, and optical (Dextrorotatory-Levorotatory or Rectus-Sinister)
For more information, google (it is now officially an English word) "isomers" on Wikipedia or Answers.com