Cholesterol is a fatty substance carried around the body by proteins. When cholesterol and proteins are combined, they are called lipoproteins. Your Cholesterol is measured it units called millimols per litre of blood (mmol/l). There are two main types of lipoproteins Low Density Lipoprotein (LDL- Bad Cholesterol) and High Density Lipoprotein (HDL- Good Cholesterol)