To be hydrophobic means to fear water. In chemistry, it refers to the property of a substance to repel water. It isn't that the substance is repelled by water so much as it has a lack of attraction to it. A hydrophobic substance exhibits hydrophobicity and may be termed hydrophobic.
Hydrophobic molecules tend to be nonpolar molecules that group together to form micelles rather than be exposed to water. Hydrophobic molecules typically dissolve in nonpolar solvents (e.g., organic solvents).
There are also superhydrophobic materials, which have contact angles with water greater than 150 degrees. Surfaces of these materials resist wetting. The shape of water droplets on superhydrophobic surfaces is called the Lotus effect, in reference to the appearance of water on a lotus leaf. Superhydrophobicity is considered a result of interfacial tension and not a chemical property of matter.
Examples of Hydrophobic Substances
Oils, fats, alkanes, and most other organic compounds are hydrophobic. If you mix oil or fat with water, the mixture will separate. If you shake a mixture of oil and water, the oil globules will eventually stick together to present a minimum surface area to the water.
How Hydrophobicity Works
Hydrophobic molecules are nonpolar. When they are exposed to water, their nonpolar nature disrupts hydrogen bonds between water molecules, forming a clathrate-like structure on their surface. The structure is more ordered than free water molecules. The change in entropy (disorder) causes nonpolar molecules to clump together to decrease their exposure to water and thus decrease the entropy of the system.
Hydrophobic vs. Lipophilic
While the terms hydrophobic and lipophilic are often used interchangeably, the two words don't mean the same thing. A lipophilic substance is "fat-loving." Most hydrophobic substances are also lipophilic, but exceptions include fluorocarbons and silicones.