In the late 20th century serious scientific studies have determined that there are only two reliable processing methods for home canning of food.
The boiling water bath, for foods with high levels of acidity, and the pressure treatment for all others. The only difference between the two methods is the use of pressure that allows us to process foods at temperatures that would be impossible to obtain with a simple boiling pot.
The “boiling water” method treat foods at a temperature of 212F (100C) while the “pressure canning” treatment can reach up to 240-250F (115-121C).
Both methods use the same basic principle. It involves placing foods in jars (usually glass) and submit them to a sustained heat treatment. This helps to destroy microorganisms that cause spoilage and the growth of harmful bacteria such as botulism. The heat treatment will also force most of the air out, creating the needed vacuum effect to seal the jars.
The choice of method is directly related to the type of food, depending on the natural level of acidity.