CT scan is a 3-dimensional X-ray. It shows up bone very well, and some other organs too. Adding contrast helps it to show up soft organs which might not otherwise be very visible on it.
A PET scan is something different, but it also includes a CT scan which the PET image is overlaid on, to make any organs containing prostate cancer show up in a different colour on the CT scan. Sometimes they're called PET/CT scans.
