Some people say it's chelation, other say citrates or lactates.
The answer really depends on which vitamin or which mineral you're talking about. For instance, some vitamins need sufficient acid in the stomach to be absorbed, while some need the presence of fat for absorption, so it's hard to make a blanket statement. You mention chelated minerals. This is a natural state for the minerals contained in fruits and vegetables. Likewise the iron in hemoglobin is naturally chelated, as is magnesium in chlorophyll.