PET stands for Positron Emission Tomography. A tomograph is a "slice". You've heard of CAT scans. That stands for Computer Assisted Tomography, where the computer generates "slice" images of a person's lungs, or brain, or abdomen. But where CAT scans show brain anatomy, PET scans show brain function.
PET scans can show which areas of the brain are active, compared to other areas, as in the example below.
For a nifty tour of image tools, and great teaching -- including the next generation of image tools called functional MRI -- go to the site that features the picture above, and scroll down their menu to the bottom of their navigation bar, to the section entitled New Imaging Techniques Show the Brain at Work. Click on the picture below.