Karma is the Law of Cause and Effect which is an integral part of Hindu Philosophy. Karma is derived from a Sanskrit word (कृ) which means - To Do. The theory of Karma is a fundamental doctrine in Hinduism. All living beings have their own deeds (Karma), thoughts, inheritance, causes, kinsman, refuge. It is Karma that differentiates beings into low and high states. Karma has a very somple philosophy - Every action has an equal and opposite reaction, that reaction is written in the cosmic registers and is helpful in determining the position of a person in the life Cycle of Birth and Death. Every action we perform or we think, it creates a cause, which in time will bear its corresponding effects. The world is made of matter, it is mysery to live in and everything we see and feel is mortal. All things comes with an expiry date. Our ultimate goal is to get liberated from this body and attain salvation. Karma helps us in deciding our direction. With Good Karma, we can get liberated and attain the Supreme Brahman and with Bad Karma, we will be flowing in this Cycle of Birth and Death.

Lord Krishna says to Arjuna in Srimad Bhagwad Gita - "To the man thinking about the objects (of the senses) arises attachment towards them; from attachment, arises longing; and from longing arises anger. From anger comes delusion; and from delusion loss of memory; from loss of memory, the ruin of discrimination; and on the ruin of discrimination, he perishes". Pleasure should not be the goal of man, but knowledge should be. Pleasure and happiness always comes to an end, it is a mistake to feel the pleasure as the goal. The only cause of all the miseries we have in the world is that we foolishly think luxury and pleasure to be the exciting situation to strive for. The world today is more stressful and a miserable place to live in. Until a person finds it glamorous, it is good to him but after a certain time we start feeling the pain of the untrue. After a time man finds that it is not happiness, but knowledge, towards which he is going, and that both pleasure and pain are great teachers, and that he learns as much from evil as from good, hence bad times are the best teachers. As pleasure and pain passes before his soul they leave upon it different pictures, and the result of these combined impressions is what is called person's "Character." If you take the character of any person it really is nothing but the aggregate of his tendencies, the sum-total of the bent of his mind. We will find that misery and happiness are equal factors in the formation of that character. Good and evil have an equal share in moulding the character of a person and in some instances misery is a greater teacher than happiness. Karma in its effect on character is the most tremendous power that man has to deal with.

The relationship of Karma to causality is a central motif in all schools of Hindu, Jain and Buddhist thought. There are some great sayings in some Hindu Sacred Texts:

As a man himself sows, so he himself reaps; no man inherits the good or evil act of another man. The fruit is of the same quality as the action.
- Mahabharata, XII-291.22

Happiness comes due to good actions, suffering results from evil actions,
by actions, all things are obtained, by inaction, nothing whatsoever is enjoyed.
If one's action bore no fruit, then everything would be of no avail,
if the world worked from fate alone, it would be neutralized.
- Mahabharata, XII-6.10, XII-6.19

Theory of Karma as a concept, across different Indian religious traditions, shares certain common themes: Causality, Ethicization and Rebirth.

Causality

Causality is the common theme to theories of Karma. Association of Karma to causality is explained beautifully in Brihadaranyaka Upanishad of Hinduism.

Now as a man is like this or like that,
according as he acts and according as he behaves, so will he be
a man of good acts will become good, a man of bad acts, bad
he becomes pure by pure deeds, bad by bad deeds

And here they say that a person consists of desires,
and as is his desire, so is his will
and as is his will, so is his deed
and whatever deed he does, that he will reap.
- Brihadaranyaka Upanishad, 4.4.5-6

As explained in the majority of Hindu Sacred Texts, Causality is the central substance in all of those. The consequence or effects of one's Karma can be described in two forms: Phal and Sanskar. This Causality theory suggests that:
1. Actions of an individual affects him and the life he lives.
2. Intentions of an individual affects him and the life he lives.

Ethicization

Ethicization is the theme of Karma referred to the consequences of an action. This consequence is to be beard in this or future lifes. Hence it is commonly said the good moral actions brings Good Karma and bad moral actions brings Bad Karma. An individual's present situation is hence explained by reference to actions in his present and previous lifetimes. Karma produces the consequence which in turn decides good and bad. Good Karma is considered as Dharma and leads to Punya (merit), while Bad Karma is considered Adharma and leads to Paap (demerit, sin).

Rebirth

Rebirth or Reincarnation is the fundamental concept of Hinduism. Rebirth is the basic cycle of Sansar and depends on Karmic merits and demerits. The rebirths and consequent lives may be in different realm, condition or form. This all depends on the Karma and its consequences. Those who breaks the Cycle of Birth and Death through Moksha reaches to the supreme reality, othewise the cycle continues in this Sansar