When you read about "heart disease" in the media, what is usually being referred to is coronary artery disease. That's just one of the issues that can come up. The heart is a fairly simple pump and it's pretty easy to break it down into components. The conceptualized heart has walls and valves, the structural part. It has an electrical system, starting in the right atrium, with a wire heading down toward the ventricles. At the connection, the wire divides into two wires, one going to the right ventricle and the other to the left ventricle. Then there's the plumbing system; the coronary arteries that carry oxygen rich blood to the heart muscle.
The plumbing system, the coronary arteries, are the part which is affected by cholesterol and the clogging thereof is the precursor to heart attacks. Of course, it's not quite that simple. Cholesterol is a generic term for several kinds of fats in the blood; complex and not well understood. Blood pressure and blood sugar and smoking and stress and genetics all have their effects on the coronary arteries as well.
A rhythm problem , like atrial fibrillation, is in the electrical system. It doesn't directly effect the coronary arteries, but can put a strain on the whole system. Sometimes the system can be shocked back into being normal again. The electrical system is studied by doing an EKG.
The valves in the heart are supposed to be one way. When the blood can be heard going from one part of the heart to another when listening with a stethoscope, that is called a murmur and is not necessarily problematic at all. Pregnant women, young children and people who are dehydrated can all have what's called functional murmurs and will often reolve on their own. Sometimes the mitral valve can have some backwards flow, but is usually not a problem. Valves can be studied by doing an echocardiogram, which is an ultrasound of the heart.
The coronary arteries are most definitively studied with a catheterization to locate and define any blockages. Usually that is reserved until there is a strong suggestion of coronary artery disease, like an abnormal stress test or EKG. All these systems interact with each other.
So if you or a loved one have been told you have heart disease, it can help to understand the differences. Any questions?