Psychedelics are a class of drug whose primary action is to trigger psychedelic experiences via serotonin receptor agonism, causing thought and visual/auditory changes, and altered state of consciousness.
The most recent "recreational" drug to be made illegal is MDMA, or "ecstasy." Its criminalization never should have happened. MDMA had a beneficial therapeutic use prior to scheduling.
Serotonin is key to understanding how antidepressants and psychedelic drugs work. Check out the infographics to learn how the serotonergic system functions and how drugs like SSRIs, MDMA and LSD interact with it.