From http://www.karmastrology.com/plfaq.shtml#pastlife

What is a past life?

It is a previous lifetime during which your soul experienced material world life in a different physical body at an earlier period of time than the present. The physical body and the personality change from one lifetime to the next, but the soul remains constant, learning new lessons with each incarnation (lifetime).

People change physical bodies from one life to the next for much the same reason that people change their underwear every day.

Reincarnational theory states that the soul enters into a series of physical forms (a specific life, such as the one you are living now) over a long period of time until it frees itself of the necessity of being in physical form. At that point the soul leaves the physical plane for the last time in order to enjoy eternal bliss (various theorists give it various names, such as Nirvana or One with Brahma--the name is not so important). It does not to return to the material plane.

Many profound volumes of metaphysical thinking from various traditions have been written on this one, and few agree. I use a fairly loose definition for this website, as it is my opinion that defining "soul" in words is probably as silly an exercise as the medieval debate over how many angels can dance on the head of a pin.

Some reincarnational thinkers, such as the Buddhists, don't even use the word soul, but prefer the concept of point of consciousness to describe that metaphysical essence which survives from one life to the next.

Others, such as the Vedic thinkers, use soul to mean a rarefied metaphysical extension of The Undefinable All which is at one with The All while simultaneously maintaining an illusion of separation from The All (In metaphysical terms, such a contradiction is apparent but not real--this stuff gets pretty cosmic in a big hurry).

At any rate, both the Buddhist and Vedic writings to me speak of the same concept in different words (I know card-carrying Buddhists and Hindus will seriously disagree with me on that one--kindly send your flames to your local priest/monk).

In any event, no serious reincarnational thinkers use the Judaic/Christian concept of the soul being a preservation of the ego personality on the metaphysical plane.

The long answer would fill several libraries, and over the eons no lack of learned and enlightened individuals have made it their life's work to write such treatises. No doubt it wasn't intended this way, but it turns out that most of these volumes are fine cures for insomnia.

Nonetheless, the short answer is that the soul, which once existed exclusively on the metaphysical plane, has desired the experience of the material plane. That wish having been granted, the soul now finds itself enmeshed in the material plane.

In dealing with that enmeshment the soul either seeks further experience on the material plane, or the soul has tired of the experience and seeks to remove itself from the necessity of further experience on the material plane. Either way the process involves many lifetimes.

Two great reincarnational traditions, Buddhism and Vedanta, do not agree on this one, and a third Eastern tradition, Sankya (out of which yoga emerged) is silent on the topic, even though all three traditions have studied the subject of reincarnation for thousands of years.

The Buddhists say, yes, we have incarnations as humans, animals, plants, and even toadstools, and that a human incarnation is something very special because it is so unusual. On the other hand, the Vedantic thinkers in the Hindu tradition quite firmly disagree, saying that once a human cycle of incarnations begins, the soul continues on with human incarnations until the cycle of incarnation is complete.

These two traditions have wasted much energy over the centuries debating this point. It would represent great spiritual progress on this planet if they recognize that they both speak to a larger truth. If there is peace, joy, and harmony, it makes little difference which path a soul chooses to enlightenment. All paths lead to the same Place.

It has nothing to do with the teachings of Jesus.

In fact, if you read the New Testament, (which few of the most strident Christians have bothered to do, BTW), it is completely silent about reincarnation. If the four Gospels (Matthew, Mark, Luke, and John) are accurate, and the born-again Bible-thumpers take great pains to assure us this is the case, Jesus never spoke of reincarnation and past lives.

In short, past lives are mentioned nowhere. This is most unusual considering how well-known the concept of reincarnation was in the Middle East two thousand years ago. It leaves begging the question of why so popular a spiritual concept be not mentioned or even referred to.

In any event, while the Bible is silent on the subject, the early Christian leaders up to the fall of Rome were lively in their debate on reincarnation, both for and against. In fact, that point of theology was quite rancorous as it led to many divisions within the early Christian church which were not easy to mend.

Many of these early arguments have not stood the test of time. For instance, St. Jerome, personal secretary to the pope 1600 years ago, argued against reincarnation because it would mean that men would have to come back into life as women and that was unacceptable. Yes, in the ancient world, such silly arguments were taken seriously.

While Rome ruled the ancient world, the Christian debate on reincarnation ebbed and flowed with no resolution, and it was not until a hundred years after the fall of Rome that the matter was put to rest by the Byzantine Emperor Justinian, who in 553 CE ordered the Fifth Ecumenical Council in Constantinople to outlaw reincarnation.

His reasons had little to do with theology. He had decided to try to revive the Mediterranean Empire but had discovered that quarrelling religious factions stood in his way. So his move against reincarnation (to bring the dissident factions to conform to his personal beliefs) was part of a larger attempt to bring order to the crumbling political and military situation of the Mediterranean in the years following Rome's collapse.

In the end, reviving the empire didn't work. But his religious edict, based on political and military expediency some 1400 or so years ago, rules Christian thinking to this day, and few Christians have been told why it is they are not allowed to look at reincarnation. It has nothing to do with Jesus, and everything to do with a dead Byzantine emperor's quest for military power following the fall of Rome.