low density lipoproteins are "particles" found in blood that contain cholesterol. Structurally, they are in the shape of a sphere with the outer boundry being composed of a single layer of phospholipid. In the core of the sphere is found esterified cholesterol. Inserted into the phospholipid shell are proteins called apoB proteins.
High density lipoproteins have a composition similar to that of low density lipoproteins with the exception that apoA protein is inserted into the phospholipid shell in high density lipoproteins.