How they work:
CT scans are like many many xrays. (Like a ring of xrays)
PET scan scans for glucose intake and causes areas/cells/parts of body that have high glucose intake to glow. (Cancerous cells have high glucose intake because they are dividing so much). PET scans can't position the results very well, they are normally done together with CT scans.
MRI Magnetizes the water molecules in the body (or something like that) and measures their response. Body parts that don't have water (such as bones), don't show up at all.
This website is amazing: