DISCLAIMER: I am not a medical or botanical expert. All of the information here is from publicly available sources, which are listed below, or is readily searchable online.
Datura is a member of the Solanaceae (nightshade) family. This family includes valuable food species such as potatoes, tomatoes, eggplants, and bell and chili peppers. It also includes toxic, alkaloid producing, dangerously psychoactive species such as deadly nightshade, henbane, mandrake, and all species of Datura. Despite the dangers of the alkaloid producing species, the member of this family that causes, by far, the most deaths is the tobacco plant.
The poisonous and psychoactive ingredients in Datura are tropane alkaloids, primarily scopolamine, atropine, and hyoscyamine. These are secondary organic products synthesized by the plants for reasons that are not fully understood. Scopolamine has been used medically as a "chemical straightjacket" for people suffering extreme agitation, and to treat Parkinson's, opiate withdrawal, and motion sickness. Scopolamine used for medical purposes requires a prescription. Atropine is used in ophthalmology to induce dilation of the pupils, in surgery to keep mucous membranes dry, to treat asthma, and as an antidote for various poisons, including nerve agents (organophosphates). Its medical uses require a prescription. Most medical grade atropine is extracted from hyoscyamine.
The alkaloids are called anticholinergic, meaning they block the neurotransmitter acetylcholine. This has a variety of effects in the human body, some beneficial for specific conditions, some dangerous or even deadly. There is a mnemonic students use to learn the symptoms of anticholinergic poisoning:
“Blind as a bat, mad as a hatter, red as a beet, hot as a hare, dry as a bone, the bowel and bladder lose their tone, and the heart runs alone.”
This refers to pupillary dilation and impaired lens accommodation; delusions, hallucinations or delirium; flushing; hyperthermia; dry mucosae and skin; gastrointestinal and bladder paralysis; and tachycardia. It is, of course, the the delusions, hallucinations, and delerium that gives Datura its spiritual kick. As with all hallucinogens, the psychoactive effects can be highly subjective, so I will not try to list them here; the next paragraph provides suggestions on finding more information.
The literature on Datura's ritual and sacred uses is vast, complicated, sometimes untrustworthy, and not possible to summarize here. Some of the best known popular sources (Castaneda, for instance) are largely fiction. The popular sources are often exaggerated and inaccurate, the scholarly sources are often dry and academic. Suffice it to say that Datura has been used shamanically for millennia for spiritual guidance, to induce visions, and for healing. The starred (*) entries in the bibliography are recommended for research into this topic; see also my Links page. A search engine query for 'datura ritual use' will return an abundance of information.