The C18 interacts with the non-polar parts of the molecule. More polar molecules elute earlier, and may not interact with the column at all.
I agree with the other 2 responders above. One of the other factors to consider is end-capping. The octadecyl C18 groups are covalently bonded to the surface of silica particles by a reaction involving silanol groups. Surface coverage of C18 on each particle is not perfect, and many free silanol groups can remain, which can easily deprotonate to give a negative charge at the particle surface (the PZC of silicates is around 2, so they are negatively charged at most pHs). This can lead to charged and induced charge interactions with analytes. The process of end-capping tries to minimize surface charge on the silica by attaching a short chain alkyl groups wherever there is no C18 coverage.
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