LSD is classified as a psychotomimetic drug (drug that induces a
psychosis-like state).
It, primarily, binds with certain subtypes of serotonin
receptors that result in net reduction in the brain's ability to
"gate" information. This means that it is no longer able to
properly filter out different sources of information. Therefore,
you get overlapping neural signals corresponding to different
sense, and in overall information processing. This results in the
random association and paraesthesias that accompany and LSD
trip.
Because it disrupts so many systems involved in information
processing, it disrupts the perception of time (temporal lobe
function), induces hallucinations (visual cortex function), and
alters one's ability to discern reality from internal stimuli (an
effect of malfunctioning "gating" mechanisms and signal overlap via
neuromodulation).
Since cascaded inhibition of the serotonin system is responsible
for this effect. Often times, administration of a serotonergic
antidepressant can reverse or prevent these symptoms.