I won't try to fill in for the many canning resources available. But the functional difference is in how hot the water can get. At sea level, water boils at 212-degrees. (At high altitudes with less pressure than at sea level, water boils at a lower temperature. That's why recipes and mixes have different high altitude directions.)
This is important because at sea level, you can't get water above 212, no matter how long you leave the pot on. No matter what the altitude or pressure, water can never get any hotter than its boiling temperature. When the pressure is greater than normal sea level, the boiling temperature is higher, which means you can get the water that much hotter. In a pressure cooker, you can get water to about 240-degrees. That matters, because the spores of the botulism can survive 212 degrees. But anything 240 and above kills them. High-acid foods don't provide an environment for botulism. But other foods do and need the high temperature treatment.
And note that pressure canning means canning in a pressure CANNER, not a pressure cooker.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen