A computerized tomography (CT) uses a computer to produce a 3D image after many 2-dimensional (2D) X-ray images are taken around a single axis of rotation. Thus, many pictures of the same area are taken from many angles and then placed together to produce a 3D image of that area (Tomography is the process of generating a 2-dimensional image of a slice or section through a 3-dimensional object. Similar to looking at one slice of bread within the whole loaf).
Unlike MRI scans, a CT scan uses X-rays. Thus it is a source of ionizing radiation which may be harmful to the patient. Thus, During the scan everybody except for the patient will leave the room. If the patient is a child, a parent might be allowed to stand nearby wearing a lead apron to prevent radiation exposure. CT is not recommended for pregnant women because there's a risk that the X-rays could harm the unborn baby.
Sometimes a contrast dye is used because it shows up much more clearly on the screen.