Samskaras are what cause human rebirth. In any other word, reincarnation.
Samskara means inborn impulse.
There are impressions, tendencies, and possibilities present in consciousness that have arisen through one's actions and thoughts, including those of earlier births. The sum total of samkaras form the person's character.
In Buddhism, spelling is Pali sankhara; generally translated as formations, 'mental formation forces' or impulses. samskara refers to both the activity of forming and the passive state of being formed.
The idea of conditioned arising says that all things--including everything with physical substance and the individual Self or soul are temporary forms. Nothing is permanent. The self, and all substantial things are simply what happens when the right elements merge together for a time. Buddhists call these elements skandas. The five skandas can be thought of as 'fields of experience' or groupings of states of existence that make people what they are.
Samskara is the fourth of the skandas and the second link in the chain of conditioned arising. Formations include all volitional impulses or intentios that precede an action. Since actins can be either physical, verbal or mental, impulses that re physical, verbal and mental are distinguished from one another. Their presence is the condition for a new rebirth. If they are absent, no karma is produced and no further rebirths take place. In addition, they determine the type of rebirth, since they can be good, bad or neutral, and their quality conditions the consciousness that arises--according to the doctrine of conditioned arising. 
In Hinduism, the Self is the Atma, the soul. The Atma is the adhara, the basis for the body and is accompanied by past karma and thoughts which surround the soul and are incorporated in the manas (mind) of the new body. Samskaras are an obstacle to enlightenment and moksha, release from the cycle of birth-rebirth.
One merits the rebirth due actions in a past life, desires, wishes and unfulfilled actions. One also merits a rebirth one's past action (karma) which merits reaction in the new lifetime.
Related: Lord Krishna teaches in the Bhagavad Gita that the mind goes forward to the next body with the atma. It is the mind, in the form of vasanas and samskaras, which create the new bodily existence.
See also, Reincarnation. Reincarnation_and_Hinduism
Notes and References
- ↑ Rider Encyclopaedia of Eastern Philosophy and Religion, page 298