Jam and jelly are made from fruit, fruit juice, sugar, and pectin. The main factor preserving these foods is the acidity of the fruit. The acidity will prevent the growth of the bacteria that causes botulism. Canning in boiling water will kill other microorganisms and seal the jars creating a vacuum. Canned jams and jellies are shelf stable.
NCHFP Resources - Types of jam/jelly, general info, extracting juice for jelly, remaking jams or jellies, includes many recipes plus low sugar versions.