Generally, the broad category known to be hallucinogens include substances that, when ingested, produce sever distortions in the perception of reality, along with changes in cognition, thought, and emotion.
One thing that most of these substances do not do, despite the name, is cause hallucinations. In reality, the perceptual distortions that are caused are known to the medical community as psuedohallucinations. True hallucinations have no basis in reality, and often look entirely realistic, while the hallucinations caused by the majority of all hallucinogens sprout from things in reality, and can usually be told apart from it.
The following are the three very broad categories that comprise hallucinogens.