I've always found that Bhikkhu Bodhi's teachings are concise, simple and accurate:
There is a tremendous variety among the living beings existing in the world. People and animals are of different sorts. What is it that causes us to take rebirth in a particular form? Does it happen through coincidence, through accident, by chance without any reason or is there some principle behind it? What is it that determines the form of rebirth we take?
Buddha answers these questions, with the Pali term "kamma". Kamma is the factor which determines the specific form of rebirth, what kind of a person we are, at the outset of our life, and it is kamma again that determines a good number of the experiences that we undergo in the course of our life.
The word "kamma" means literally action, deed or doing. But in Buddhism it means specifically volitional action.
The Buddha says:
"Monks it is volition that I call kamma. For having willed, one then acts by body, speech or mind". What really lies behind all action, the essence of all action, is volition, the power of the will. It is this volition expressing itself as action of body, speech and mind that the Buddha calls kamma.
This means that unintentional action is not kamma. If we accidently step on some ants while walking down the street, that is not the kamma of taking life, for there was no intention to kill. If we speak some statement believing it to be true and it turns out to be false, this is not the kamma of lying, for there is no intention of deceiving.
Kamma manifests itself in three ways, through three "doors" of action. These are body, speech and mind. When we act physically the body serves as the instrument for volition. This is bodily kamma. When we speak, expressing our thoughts and intentions, that is verbal kamma, which can be performed either directly through speech or else indirectly through writing or other means of communications. When we think, plan, desire inwardly, without any outer action, that is mental kamma. What lies behind all these forms of actions is the mind and the chief mental factor which causes the action is the volition. GO TO http://www.beyondthenet.net/dhamma/kamma.htm
for the following links:Every choice of our's has a tremendous potential for the future
Kamma is like a seed
Type of Kamma Based on the Time of Fruition
Types of Kamma based on Ethical Grounds - Wholesome and Unwholesome Kamma
Why is one intelligent and another dull minded? How is one born ugly and another beautiful?
Survey of Buddhist Cosmology
Mind is the architect of the whole universe
We are not hopeless prisoners of our past
Going beyond kamma - the ultimate aim of the Path