In general, lipids can be found in cell membranes and in fats. Cell membranes are composed of a double layer of lipid molecules. Most membranes in cells are semipermeable , allowing some things to easily pass through and blocking other things. Sometimes the cell can control this process and other times it cannot. The energy involved in this process has both chemical and physical aspects and is far from completely understood.
Fats are esters formed from glycerol and fatty acids.
Fatty acids are carboxylic acids with 12 to 20 carbon atoms in the chain. In general, animal fats are more saturated and plant oils are more unsaturated. Steroids are another class of lipids. All steroids contain a tetracyclic ring system. One steroid produced by your body is cholesterol.