"We eat what we can and freeze what we can, and what we can't, we can" is a saying in my family.
The USDA Complete Guide to Home Canning is the bible of safe canning. If it's done right, canned food can still be safe to eat after 100 years.
Io demonstrates at "Fort Awesome" in Berkeley CA. Other illustrations are from the USDA
Step 1: Why Canning?
This chart from the USDA shows why this is good advice. The microorganisms that cause food to spoil don't live well at high and low temperatures.
Canning is a way to preserve food at room temperature. It works by cooking the food and containers at high temperatures to kill micro-organisms and sealing the jar so no new ones can enter.
Properly canned food is safe. Improperly canned food can cause Botulism poisoning from Clostridium Botulinum bacteria. The name comes from the Latin word for "sausage", "botula".(wpedia)
The spores of this bacterium are present nearly everywhere. They can survive some boiling. They thrive in an anaerobic environment such as a sealed can, producing a nerve toxin. They can't handle acidity below ph4.6, oxygen, or a wet temperature above 250f.
The keys to safe canning of food are PH, moisture content, cooking temperature, pressure, time, sterile procedures and proper sealing.