I have used karma in a previous A to Z challenge but it is a topic that I have a deep connection to. I have a deep belief that what you put out into the world does come back to you and I try to live my life as much within that philosophy as I possibly can. I don't want to put anything out into there into the world that's going to come back to bite me in the butt. It gets a little easier with age.
The theory is that good intent and good deeds lay the way for good karma and future happiness whereas negative intent and bad deeds create bad karma and future suffering. In some religions it is tied to the concept of future lives so what you do in this life effects what happens to you in future lives.
Karma is difficult to define because there is so much diversity in the views of the different religions but it does seem to share common themes. One of the main ones being that like deeds cause like effects, the effects of karma are not necessarily immediate but can effect one later in life.