Karma is a concept in Asian religions such as Hinduism and Buddhism that are sympathetic to the idea of reincarnation. It is held to represent the "balance" of good/bad acts we have performed in this lifetime and others.
Performing meritorious acts results in rebirth into a higher station, such as a superior human being or a godlike being, while evil acts result in rebirth as a human living in less desireable circumstances, or as a lower animal. While the action of karma may be compared with the Western notions of sin and judgment by God or gods, karma is held to operate as an inherent principle of the Universe without the intervention of any supernatural being.
In Sanskrit, the word Karma (spelled several ways) is defined as action. There is an understanding implied that action causes re-action. The re-action, of course, is also an action, ad ifinitum. There is no belief in the earliest definitions of karma that there is any kind of punishment attendent on actions that are performed. However, it was also believed that Intent is an action in itself, and could be even more powerful than a physical action.
This stirring of the pot, so to speak, is unavoidable if one is to be alive, eating, breathing, etc. However, in the Buddha's teachings as well as in Vedanta and Shaivism, one is advised to avoid desire and aversions as a way to have less involvement with Karma.