Atherosclerosis refers to the buildup of fatty plaque on the interior lining of the arteries — the medium- and large-sized blood vessels that carry nutrients and oxygen to tissues throughout the body. The plaque is made up of fats, calcium, and more. The formation of these plaques damage the inside surface of the arteries and leads to more plaque buildup, inflammation, artery hardening and narrowing, and an increased risk of clotting. Severe plaque buildup leads to a wide range of diseases, including coronary artery disease, carotid artery disease (blockage of the arteries feeding into the brain), and peripheral arterial disease (blockage of arteries in the arms, legs, or pelvis).
Atherosclerosis should not be confused with arteriosclerosis, which refers to general thickening and stiffening of blood vessels of all sizes.  Atherosclerosis can be considered a type of arteriosclerosis since it can ultimately lead to stiffer and thicker blood vessels.
Atherosclerosis can be visualized using a process called angiography. While this is one of the better methods for assessing atherosclerosis, it’s not usually used for routine screening since it’s invasive. Other less invasive methods can be used to assess atherosclerosis, including coronary calcium scans that measure calcium buildup inside heart arteries (less invasive, but involves radiation exposure) and the ankle-brachial test (noninvasive; mainly relates to atherosclerosis in the peripheral arteries, but atherosclerosis in the periphery is correlated with atherosclerosis elsewhere in the body). 
Atherosclerosis isn’t usually diagnosed directly through imaging or the other measurements mentioned above. Instead, it’s either diagnosed through the criteria for the clinical diseases it leads to (e.g., peripheral artery disease) or assessed on the basis of risk factors such as biomarkers like high blood pressure or LDL-C, demographics like age, or behaviors like smoking.
Medical treatment options depend on the type of clinical disease and how far it’s progressed.
If a person doesn’t have clinical disease yet, then the main medical treatments usually target the major risk factors of high cholesterol and high blood pressure using drugs like statins and antihypertensives as spelled out in major guidelines.
Medical treatment is often added on top of lifestyle changes, since lifestyle plays a large role in atherosclerosis. The major lifestyle recommendations are discussed more below.
Yes, although exactly which supplements qualify depends on whether you count studies involving markers tightly related to atherosclerosis like LDL-C or only count studies that measure atherosclerosis more directly. A lot more studies have looked at biomarkers related to atherosclerosis than direct measurements. However, since a lot of studies have examined biomarkers like LDL-C and blood pressure that are firmly established to actually cause atherosclerosis, the evidence base isn’t too bad.
A very incomplete list of supplements examined for impact on atherosclerosis or its biomarkers include:
Omega-3 and omega-6 polyunsaturated fatty acids
Garlic and allicin (one of garlic’s main bioactives)
Antioxidant vitamins such as vitamin C and vitamin E
Diet plays a huge role in atherosclerosis through impacting many of its associated risk factors, such as obesity, high cholesterol, high blood pressure, and diabetes. Recent guidelines cite strong evidence that weight loss in people with overweight or obesity can reduce atherosclerotic disease risk. There’s also strong evidence that a diet high in fruits, veggies, legumes, nuts, whole grains, and fish can reduce the risk for diseases caused by atherosclerosis. Weaker evidence suggests that swapping out saturated fat with mono- or polyunsaturated fats and minimizing processed meats and refined carbohydrates can also help.
Two of the biggest lifestyle changes beyond diet that can impact atherosclerosis are quitting smoking and getting aerobic exercise. To reduce atherosclerotic risk, people should ideally perform at least 150 minutes of moderate-intensity aerobic activity, 75 minutes of vigorous-intensity aerobic activity, or a combination of the two each week. A rule of thumb to measure intensity is the talk test: moderate aerobic activity makes it hard to sing but not to talk, whereas it’s hard to say more than a few words at a time during vigorous aerobic exercise.
Quite a few factors are needed to cause atherosclerosis, the main ones being damage to the artery lining, fats, and inflammation.
Atherosclerosis starts with damage to the interior lining of medium and large arteries, which often occurs in arteries that are under high pressure and tension, a process that can start in childhood. This damage leads to the fats — which are mostly in the form of low density lipoprotein (LDL) — being trapped in the interior arterial lining. The fats then get oxidized, which attracts white blood cells — mainly macrophages — to try to clean up the mess. However, when there’s a lot of fat to clean up, the macrophages overdo it and transform into foam cells, which is one of the main ingredients of atherosclerotic plaque formation.