A pressure cooker is most often about the size of a small stock pot, a pressure canner is made to fit jars inside and the most common sizes fit either one layer of quarts, or two layers of pints. I think they are the same except for size so you can use a pressure canner to pressure cook larger volumes of food. I've only used my pressure canner to can, but I think I read in the booklet that it can also be used to cook.
I think you could use a cooker to can if you have the proper weights (different foods can at different weights of pressure), there is a metal disc on the bottom of the pot to help diffuse the heat, and the jars fit snug enough that they don't rock around and break while in the canning process.