According to current scientific understanding, souls are energy patterns emerging from or through the Astral Plane which attach to living beings at some point after conception but prior to birth. The exact time when the soul forms a symbiotic bond with the living host seems to be somewhat variable and the precise factors influencing this are currently being studied. In rare cases, the creature is born (or hatched, in the case of egg-laying creatures) without any soul at all, in which case it usually lacks any empathy or strong emotions. Such beings frequently end up being possessed by extraplanar entities such as devils, demons, or angels, as the body has no resistance to possession without a soul that already inhabits it.
Generally, the soul remains joined with the body for the rest of the life of the creature, excepting rare (and dangerous) phenomena such as astral projection. Major but non-lethal damage to the body, especially to the brain and certain other vital organs, may damage the connection to the soul, resulting in the partial or complete loss of emotions or memories. Certain rare diseases have similar effects. Should the damage be healed (usually magically), the connection to the soul likewise regenerates.
Upon the death of the creature, the soul generally detaches. However, a spiritual bond remains to the corpse for a duration averaging about a month, which permits ritual magic to restore the body and reattaching the soul, resurrecting the creature. Certain necromantic rituals can preserve the bond for longer time periods or even cause the soul to possess the corpse, producing an undead creature such as a lich. Even if the soul is not reattached, some spiritual residue remains in the corpse, which allows rituals to animate the corpse as a zombie, skeleton or similar creature as this residue contains memories of the reflexes needed to move the body - making animating a corpse much easier than animating a construct, as the latter has no spiritual "memories" of ever having moved before.
After death, most souls move on to the domain of their deity, the Road to Nowhere, or directly to the Great Beyond, although some remain as ghosts in the Material Plane or the Plane of Shadows. These souls remain their memories and (usually) the same personality as they had in life, but it appears that without a living body to bond with they have a hard time learning new things or otherwise evolving (sapient corporeal undead, such as liches and vampires, have similar but lesser difficulties with learning).
However, some souls are sucked back to the Material Plane and bond with new unborn creatures, thus reincarnating into new lives. This phenomenon is most well-known with the nephilim as well as with the natives of the subcontinent of Ranya, but there are enough credible accounts of other instances to show that this is a far from uncommon occurrence. Usually (with the exception of the nephilim) such people retain no conscious memories of past lives, but they may nevertheless experience dreams and strange flashes of déjà vu. Magical rituals exist to facilitate experiencing memories of past lives, but it should be said that most mages offering them are charlatans using illusions and mind-altering magics to con the gullible by encouraging them to believe that they are the reincarnations of famous historical people. In rare cases, the soul actually somehow splits in half, causing each part to reincarnate into a different body. This causes the phenomenon called "soul mates" where each person will feel "incomplete" without their other "soul half" and constantly seek out their counterpart. However, this phenomenon is not nearly as common as portrayed in popular fiction, and neither is it assured that each half will end up being in a body of a different physiological sex.
Regardless, the vast majority of people do not seem to have past lives, as far as it can be determined magically. This raises the question of where their souls come from, and current scientific opinion is divided on whether they are spontaneously generated on the Astral Plane, they reincarnate from more primitive life forms (and some people with past lives do have memories of being animals, once…), or whether there is some divinity, cosmic entity or force generating them.