Karma is a universal law of action and reaction. "Do unto others as you would have them do unto you" and "what goes around comes around" partially describe how karma works—for whatever action we do, there's a reaction. Our future depends on how we act now.
Our karma, to a large degree, dictates our lot in life (rich/poor, educated/uneducated, unattractive/beautiful, etc.). It's also what forces us to be born and die perpetually. "Good" karma (we do good works and enjoy material happiness) or "bad" karma (we perform immoral or irreligious work, and suffer later on) are both bad, because to enjoy or suffer we have to be born and die again in future material bodies. But anyone completely surrendered to the will of God is free from the effects of karma.
- From our Overview: What is Karma and how does it work?
- Article: Bad Karma – there's more to it than stubbing your toe, by Jayadvaita Swami
- Article: What is Karma? by Stephen Knapp
- Q and A session: Karma FAQ, from The Law of Karma – A Course in Vedic Knowledge, with Pavanesana Das
- References to Karma in Vedic literature