Every action generates a force of energy that returns to us in like kind... what we sow is what we reap. And when we choose actions that bring happiness and success to others, the fruit of our karma is happiness and success.
Karma is experience, and experience creates memory, and memory creates imagination and desire, and desire creates karma again. If I buy a cup of coffee, that's karma. I now have that memory that might give me the potential desire for having cappuccino, and I walk into Starbucks, and there's karma all over again.
Karma, memory, and desire are just the software of the soul. It's conditioning that the soul undergoes in order to create experience. And it's a cycle. In most people, the cycle is a conditioned response. They do the same things over and over again.
The "Bhagavad Gita" is actually a very good text for yoga - the yoga of love, the yoga of action or karma, the yoga of understanding of intellect, and the yoga of reflection and meditation. I think it's a very important map for understanding the nature of consciousness.
In every conversation I've had - with housewives in Mumbai, with middle-class people, upper-class, in the slums - everyone says there is an underlying consciousness of karma. That people believe in karma - that what you're putting out is going to come back. If I do something to you, the energy of it is going to come back to me in the future.
Karma means your have to live with the consequences of the actions you have taken in the past. Whatever you put out is coming back.