What is meant by bad and good cholesterol

Cholesterol is a lipid molecule  biosynthesized by all animal cells and is an essential structural component of all animal (not plant or bacterial) cell membranes  required to maintain both membrane structural integrity and fluidity. It plays a vital role in how every cell works and is also needed to make Vitamin D, some hormones and bile for digestion. However, too much cholesterol in the blood can increase risk of getting heart and circulatory diseases. Cholesterol can't dissolve in the blood. It must be transported through the bloodstream by carriers called lipoproteins.

The two types of lipoproteins that carry cholesterol to and from cells are low-density lipoprotein, or LDL, and high-density lipoprotein, or HDL. LDL cholesterol and HDL cholesterol, along with one fifth of your triglyceride level, make up your total cholesterol count, which can be determined through a blood test. Total blood (or serum) cholesterol is calculated using the following equation: HDL + LDL + 20 percent of your triglyceride level. 

LDL (Bad) Cholesterol

LDL cholesterol is considered the “bad” cholesterol because it contributes to plaque, a thick, hard deposit that can clog arteries and make them less flexible. This condition is known as atherosclerosis. If a clot forms and blocks a narrowed artery, heart attack or stroke can result. Another condition called peripheral artery disease can develop when plaque build up narrows an artery supplying blood to the legs.

HDL (Good) Cholesterol

HDL cholesterol is considered “good” cholesterol because it helps remove LDL cholesterol from the arteries.  HDL acts as a scavenger, carrying LDL cholesterol away from the arteries and back to the liver, where it is broken down and passed from the body. One-fourth to one-third of blood cholesterol is carried by HDL. A healthy level of HDL cholesterol may also protect against heart attack and stroke, while low levels of HDL cholesterol have been shown to increase the risk of heart disease.