Computed Tomography is a specialized X-ray imaging technique.  It may be performed "plain" or after the injection of a "Contrast Agent".  CT creates the image by using an array of individual small X-Ray sensors and a computer. By spinning the X-Ray source and the sensor/detectors around the patient, data is collected from multiple angles. A computer then processes this information to create an image on the video screen. These images are called "sections" or "cuts" because they appear to resemble cross-sections of the body. This technique eliminates the problem of conventional X-rays, where all the shadows overlap. Don't worry though, the patient is not actually "cut".
 Because it does use X-Rays to form the image, this computerized technique has some limitations that are similar to those for plain film radiographs.