Are Lipids Polar or Nonpolar?

Answer: Lipids are typically nonpolar due to the large number of nonpolar covalent carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds present within the molecule. These result in structures that exemplify nonpolar characteristics even though they may contain one or two groups that are polar by themselves. 

As the diagram below indicates, there are a few different types of lipids including cholesterol, fatty acids, triglycerides and phospholipids. There are also different kinds of waxes and vitamins that are classified as lipids. The specific naming and classification scheme for different lipids depends on the specific configuration of the chemical bonds within the lipid. Some lipids (ex. phospholipids) have a hydrophobic tail and hydrophilic head. This allows one part of the molecular to be nonpolar (tail) and another to be polar (head). This allows for the formation of lipid bilayers - a similar property present in soap. However, lipids typically only refer to nonpolar molecules in biology.

Different kinds of lipids and their structural formulas. Source 

What purpose do lipids serve within biological contexts?

There are a wide range of functions associated with lipids. As hinted at earlier, phospholipids form the bilayer that separates the internal cell environment from the outer environment and permits organelles in eukaryotic cells to compartmentalize functions and create special environments within the cell. Furthermore, hormone lipids and vitamins play an important role in cell signaling and transduction cascades. For example, metabolism, reproduction and blood pressure regulation mechanisms involve different hormones. Lastly, the most recognizable use of lipids is for energy storage in the form of fats. This is effective because fatty acid triglycerides can store a large amount of energy within small cell vesicles.