CT scan is performed by using an x-ray beam passing through the body to produce an image. The Computed Tomography scan x-ray beam spins around like a saw blade in the circular part of the scanner called the gantry. You lay on a table that moves in and out of the gantry while the table passes you through the spinning x-ray beam.
This abdominal scan illustrates how it works. Imagine slicing a loaf of bread for a sandwich. The knife would represent the x-ray beam that the scanner uses and the loaf of bread would be your body.
Another good example is a spiral cut ham. Suppose you went to the deli section at the grocery store, and asked the butcher for a pound of ham. You would see him put the ham on the meat slicer tray and ask you how thick you want it sliced. The blade on the meat slicer would represent the x-ray beam cutting through the body to produce the images. I promise you, you can not feel an x-ray beam passing through you.
The "a" in cat scan has been dropped and is now known as ct scan.