When preserving products you need to discern if the product to be preserved is acidic or non acidic. The majority of fruits are acidic, whilst the majority of vegetables are non acidic. Some borderline foods can be made acidic through the addition of lemon juice or vinegar.
Acidic foods can be preserved by waterbath canning: this process brings products up to boiling temperature for a specified period to kill moulds and enzymes. This method is recommended for most fruits, jams, chutneys and jellies.
Non acidic foods cannot be preserved in a waterbath because bacteria will thrive in a non acidic environment and temperatures need to exceed boiling point to kill off bacteria. This can be achieved by the use of a pressure canner which is a specialised pressure cooker for food preserving. Pressure canning should be done using tested recipes and if in doubt always err on the side of caution and treat your food as non acidic and preserve by pressure canning rather than waterbathing.
Waterbaths are large vessels which can be boiled either on a stove top or via an electric device. They include a rack to keep the jars off the bottom of the canner.
Pressure canners come in 2 basic types: ones such as the Presto canner which use a rubber gasket similar to a standard pressure cooker and the All American pressure canner which uniquely uses a metal on metal seal.