RIG VEDA - YAJUR VEDA - SAAM VEDA - ATHARVA VEDA

Law of Karma
God is All-blissful, Formless, Almighty, Just, Merciful, Unborn, Infinite, All-pervading, Omniscient, Controller of All from within, Immortal, Fearless, Eternal, Pure and Creator of the Universe.
Souls are numerous. They are neither created nor ever cease to exist. The physical world is made of matter. Matter always exists and never gets destroyed, it only changes shape. God creates the universe from matter for the benefit of souls. Without creation of the universe, the souls are in deep sleep and as good as dead. Everything we see, feel, hear, touch, smell is made of matter. All bodies are also made of matter. When the soul joins the body, it is called birth and when the soul separates from the body, it is called death.
Humans are the only species who perform good or bad deeds and reap the rewards or punishment from their deeds in done previous and present human life. Deeds performed by humans never go un-rewarded or un-punished. Animals on the other hand, do not do any deeds. They are rewarded or punished with the life and the environment they are given. This is called the Law of Karma.
God is Just, therefore souls are born either human or animal, based on the deeds they performed when in human life. Based on that, one person is born with all the luxuries and the other is born in extreme poverty. Furthermore, a good person (soul) is born again as a human being after his/her death, but a bad person (soul) endures millions of lower lives (as animals, birds, insects, etc.) before being reborn as a human being.
Moreover, no life, human or animal, is without suffering. The real purpose of human life is to get rid of the cycle of birth and death. Escaping the cycle of birth and death is called Moksha. Moksha is the highest spiritual point, when no birth or death occurs, and the soul enjoys Godly bliss. Moksha is possible only thru piety and meditation.