What are hydrophobic molecules (or hydrophobic molecular regions)? What are hydrophilic molecules? How can they be characterized in relation to their polarity?

Hydrophobic molecules are those that have little or no propensity to dissolve in water (hydro = water, phobia = fear). Hydrophilic molecules are those that have great propensity to dissolve in water (philia = friendship).

Water is a polar substance. Remembering the rule that "equal dissolves equal" one can end that hydrophobic substances are non-polar molecules while hydrophilic molecules are polar molecules.