Coronary artery disease is usually caused by the build up of a fatty substance called plaque within the wall of the coronary arteries. The scientific name for this substance is "atheroma", which comes from the Greek for "gruel" or "porridge"!
This build up within the walls can over time can cause a reduction in blood flow down the artery. In some patients this can get to quite an advanced stage before symptoms arise. The usual symptoms which patients feel are chest pain or tightness, or breathlessness. It is important to understand, however, that this build up usually occurs over many years, and there is a long "pre-clinical" stage where there is a build up of atheroma but no symptoms. With this build up atheroma comes a risk of heart attacks; most heart attacks occur in patients who have not previously had symptoms. This is why it is so important to understand your heart health, and look after yourself with a sensible diet, plenty of exercise and, when appropriate, preventative medications.
To investigate this further, a number of tests are available. Each has advantages and disadvantages, and your G.P. or cardiologist will be able to discuss these with you.