Well, it's a treadmill test to see how your heart reacts to strenous exercise. There are several different ones. If you google stress tests you can read about them. Basically they hook you up to an EKG and monitor your blood pressure and oxygen levels and what your heart does when you exercise. They have you start out by walking normally and slowly increase the speed and incline until you're pretty much running. They want you to get up to your target heart rate. If at any time during the test you feel you need to stop then they stop. I'm actually thinking of having a Thallium stress test rather than a Cardiac CT Scan as I've researched them and the thallium test seems more accurate at detecting heart problems. Try a google search to get more info
Hope that helps! You could also visit the heart disorders board they have a lot of info over there too!
Take Care,
Tori