Let's start to explain a CT scan by explaining what an x-ray is.  An x-ray, is a single view of anatomy that is developed by using radiation at different strengths to make different radiographic images.  When x-rays are taken they expose the tissue to create an image of grey scale depending on tissue attenuation. 

A CT scan is multiple x-rays taken in 360 degrees around a body part every 1 mm.  This stretch of multiple radiographs are interpretated on an advanced computer program to create the patients internal anatomy.   

Here is a video explaining how a CT scan works in human medicine.  



In veterinary medicine a CT scan will be recommended by your local vet when a problem arises that is difficult to image.  The first line technology for veterinarians is to run bloodwork, then radiographs, and an ultrasound.  If the problem cannot be identified with the first line of testing then a CT may be the next recommendation to try and identify or rule out the problem.