Following Albrecht Kossel's original proposal in 1891, natural products are often divided into two major classes, the primary and secondary metabolites. Primary metabolites have an intrinsic function that is essential to the survival of the organism that produces them.  Secondary metabolites in contrast have an extrinsic function that mainly affects other organisms.  Secondary metabolites are not essential to survival but do increase the competitiveness of the organism within its environment.  Because of their ability to modulate biochemical and signal transduction pathways, some secondary metabolites have useful medicinal properties.
Natural products especially within the field of organic chemistry are often defined as primary and secondary metabolites.  A more restrictive definition limiting natural products to secondary metabolites is commonly used within the fields of medicinal chemistry and pharmacognosy.