You can only ask one question at a time so I edited down accordingly.  People at high risk for coronary artery disease have various risk factors including poor diet, smoking, and family history.  One major risk factor is elevated levels of cholesterol.  Chemically, cholesterol is a fatty steroid alcohol.  Like other steroids, it has four fused ring systems (three six membered rings and one five membered ring) called the A, B, C, and D rings.  The B ring has a double bond, and A ring has a hydroxyl group (alcohol), and the D ring contains an aliphatic hydrocarbon chain.

Since cholesterol is very non-polar, it solubility in water in minimal.  This means that its solubility in blood is also minimal.  As a result, cholesterol is transported through the bloodstream by lipoproteins, polymers that contain lipids (fats) and proteins.  The density of these lipoproteins determines the amount of cholesterol that they can carry.  LDL (low density lipoprotein) carries a higher percentage of cholesterol while HDL (high density lipoprotein) carries a smaller percentage.  So HDL in considered the good cholesterol and LDL the bad cholesterol.  But the actual cholesterol itself in the lipoproteins is the same no matter the density level.  Chemically there is no difference between the two.