There are many different kinds of lipids; the most common molecules called by that name are fats in the body, but "lipid" covers vitamins, fats, and waxes, among others. The only thing they all have in common is that they are all hydrophobic, or insoluble/insolvent to water.
Lipids are important in the body for storage of energy, and their aversion to water makes them uniquely suited to that task. If they were hydrophilic, or water-soluble/solvent, the fatty acids inside could be absorbed into the bloodstream and filtered out in the kidneys. Instead, they are created and processed inside the adipose tissue and show a higher caloric content then carbohydrates and proteins, both of which are water-soluble. All lipids have different molecular structures which allow them to perform different functions; the hydrophobic aspects that they all share allow them to gather together and form more complex structures; otherwise, they could dissolve completely in water.