Lipid droplets are lipid stores inside cells that are surrounded by a phospholipid bilayer and are often found insidide adipocytes. Lipid droplets are able to store neutral lipids e.g. triaclycerides and cholesterol esters that are synthesised in the endoplasmic reticulum. The lipids stored inside the droplets are neutral due to the fact they contain no hydrophilic head groups, instead all containing hydrophobic constituent molecules, that clump into droplets rather than bilayer (as would happen if they contained hydrophilic heads). 
Lipid droplets form when neutrally charged lipids become depositied betweent the layers within the endoplasmic reticulum membrane. Once deposited, the neutral lipids clump together to form a 3-D droplet, which buds off of the endoplasmic reticulum membrane and separates from the E.R. The lipid droplet is surrounded by a phospholipid bilayer that contains numerous proteins associated with lipid metabolism.