If you use the correct canning method for the type of food you want to preserve, you will happily and safely preserve jars of delicious food for your pantry. However, if you mismatch the food and the canning method things could get scary (think botulism). Fortunately, it's really easy to get this right and dive into totally safe, worry-free canning...once you understand two simple things.
The first thing to understand is that there are two different kinds of canning. One is boiling water bath canning, which requires no special equipment beyond the canning jars. The other is pressure canning, which requires a very specialized piece of equipment called a pressure canner (no, that's not the same thing as a pressure cooker).
Boiling Water Bath
A boiling water bath is simply a large pot (you can use a stock pot) with a rack on the bottom. Canning jars filled with food and with special canning lids secured are completely immersed in boiling water for an amount of time specified in the canning recipe. After processing, as the jars cool, a vacuum seal is formed. A boiling water bath can only heat the food to the temperature of boiling water.
A pressure canner is a heavy-duty piece of equipment with a vent, a pressure gauge, and screw clamps. It is capable of heating the food in the jars to hotter than the temperature of boiling water.
The second thing to understand is which foods can be safely processed by which method. Here's the basic rule: all low acid a.k.a. alkaline foods must be processed in a pressure canner, not a boiling water bath. What does that mean? It means that any unpickled vegetable, including vegetable soup stocks and all animal products, cannot be safely processed in a boiling water bath. You need a pressure canner for them.
The reason for that is that although botulism bacteria is killed at the temperature of boiling water, botulism spores can survive that temperature. The spores can be eliminated by temperatures hotter than boiling water, which requires a pressure canner, or by creating an extreme pH (as is the case with vinegary pickled foods and sweet preserves).
Acidic and Non-Acidic Foods
Vegetables in plain or lightly salted water and animal products have a fairly neutral or slightly alkaline pH. Because the pressure canner creates temperatures hotter than boiling water, it can be used to process these non-acidic foods.
All acidic foods - fruits, pickled vegetables, sugar preserves, and tomatoes with a little-added acidity (lemon juice, vinegar, or citric acid) - may be safely processed in a boiling water bath. In boiling water bath canning, it is the acidity of the ingredients as much as the heat of the processing that safely preserves the food.
There is one other thing about canning that sometimes confuses people, and that is the word "canning" itself. For starters, we don't usually use cans, as in metal cans, for home food preservation anymore. We use glass jars, a fact that has led some enthusiasts to call the process "jarring." But jarring reminds me of something that is a harsh or abrupt jolt, so I'll keep using the word canning even though it isn't strictly accurate.