Some naturally occuring tryptamines are neurotransmitters (e.g. serotonin, melatonin and bufotenin), but most are found in plants, fungi and animals and act predominantly as psychedelics.
Classical psychedelics all activate serotonin 2A receptors to produce their primary effects on perception and behavior. Some examples of simple tryptamines include psilocin and DMT, whereas LSD is considered a complex tryptamine (or an ergoline).
The similarity of psychedelics with endogenous tryptamine, serotonin can be seen below (Nichols 2012). They all share an indole ring and the side chain amine group, which are modified to produce different psychedelics.
See our video below in how these tryptamines share some structural features but differ in others: