Lysergic acid diethylamide, more commonly known as LSD, is a non-toxic, non-addictive molecule which mimics serotonin in the brain.  The body 'mistakes' LSD for serotonin and shoots it across the synaptic cleft.  LSD has a higher affinity for 5-HT receptors than serotonin, thus the presence of LSD prevents serotonin from sending neural messages in the brain.  Once the LSD molecule is bound to the receptor proteins the message is not carried any further.  Instead the impulse is redirected to the older parts of the brain, where the bloodstream then takes it to the sense interpretive centres and the motor areas.

http://faculty.washington.edu/chudler/lsd.html  

There are many similarities between the molecules of serotonin and LSD which allows this process to occur, the most obvious being their close structural similarities, particularly the indole ring shown highlighted in blue.