What is atherosclerosis? How is the condition treated and what is the outlook for people who have been diagnosed with atherosclerosis?

Atherosclerosis is a disease of the arteries .
It happens when fat gets deposited in the inner walls of the arteries, restricting the flow of blood through them, as the lumen (space inside) of the artery becomes narrower.
The deposited fat is called plaque .
But the deposition dosen’t just stop there. If not treated, this can turn into something much dangerous.
The plaque keeps on building up and the lumen gets narrower. At one point it gets so narrow that a blood clot forms and blood stops flowing through the artery. If this happens in an artery that supplies blood(and hence oxygen) to the heart muscle(coronary artery), a heart attack happens ##-## in technical terms called a myocardial infarction, which can cause death.
Atherosclerosis is treated in three ways:
Lifestyle changes: Maintaining weight, regular exercise etc. And regulating diet and food intake, quitting smoking.
Medicinal drugs treatment
Surgical procedures such as PCI and CABG. More on that (here).
The outlook varies. Some people survive for years with less blood flow to vital organs such as the heart. But some die suddenly due to a heart attack. The important thing is to control the risk factors of the affected individual unique to that person.