Obesity and overweight are strongly linked to elevated blood triglyceride levels, elevated LDL (bad) cholesterol levels and reduced HDL (good) cholesterol levels. These blood-fat abnormalities all increase risk for cardiovascular disease including heart disease and stroke.
As I have described, the linchpin to a lot of the risk associated with obesity is what is called metabolic syndrome which is the name we now give to the combination of obesity, visceral fat accumulation, pre-diabetes , generalized (throughout the body) chronic inflammation, hypertension and the above-described lipid abnormalities. Interestingly, not all overweight or obese people have metabolic syndrome just as not all thin people escape it.
Cholesterol has the invidious distinction of being the most famous (or infamous) of all blood fats. Nearly everyone understands that there is something "bad" about cholesterol. This common-knowledge derives from one of the greatest medical studies in human history, the "Framingham Heart Study". Anyone who was old enough to remember the the 1960's, 70's and 80's will likely also remember the great attention that was paid by society, doctors and the news media to the dangers of smoking, obesity and cholesterol. ALL of these warnings derived from the Framingham Heart Study and nearly all of them are as valid today as ever.
Cholesterol itself is a lipid (water insoluble chemical) that is very closely related to and is a precursor of a number of important hormones like testosterone. Cholesterol is necessary for human health but in small amounts where it appears to help our cells control the integrity of their "cell membranes". High levels of cholesterol in the blood are another matter and increase risk for "atherosclerosis" or scarring of arteries caused by accumulation of cholesterol deposits in artery walls. This scarring can partially occlude (block) arteries and because the scarring and underlying cholesterol crystals is/are sharp, they can damage red blood cells as they flow past and lead to a blood clot in the artery. Sometimes, often in fact, such a clot will cause sudden total blockage of an artery. If the blocked artery happens to be a coronary artery (one of the heart's "feeder arteries"), then the event leads to a "mycardial infarction" (M.I.) or heart attack.
By the 1980's, and again thanks to the Framingham Heart Study, we came to understand that the "cholesterol story" wasn't quite a simple as I described above and that cholesterol tends to occur in blood not as native, free-floating dissolved molecules, but as very "large" agglomerations of cholesterol, other fats and protein. These agglomerations are called "lipoproteins" (fatty proteins) and it turns out the the really dense ones (the ones that settle fastest to the bottom of a tube in an ultracentrifuge) actually protect AGAINST hardening of the arteries and heart disease. Conversely, the cholesterol lipoproteins that "float" to the top in an ultracentrifuge are the "bad" lipoproteins that increase heart disease risk. This is why we now called the dense lipoproteins "high density lipoprotein" or "HDL" and the light ones are called "LDL". To reiterate, "HDL" is "good" and "LDL" is "bad".