Most Christians don't believe in karma, or at least, they say they don't. Perhaps because the name comes from Hindu and Buddhist teachings, and is often associated with reincarnation, which most Christians don't believe in, they think they shouldn't believe in karma either. In truth, Christians do believe in karma, just not by that name.
    One of the first things we are told in the Bible, is that when Adam & Eve disobeyed God and ate from the tree of Knowledge of Good & Evil (NOT an apple tree), they were kicked out of the garden: karma.  Their son, Cain, was banished for killing his brother: karma.
    Karma is said to be cause and effect, but not so much in the way that it is defined in science, but in a more spiritual way. Essentially, it means that your acts and thoughts, in the past and in the present, have an effect on what will happen to you in the future. If you do good things and have kind, loving thoughts, you will earn good karma, meaning good things will happen to you in the future. If you behave badly and are filled with thoughts of anger, jealousy, and other negative things, your future will not be so pleasant. 
    In Christianity, it is not very different. In Christianity, when you sin, you must suffer and be punished and when you do good things you will be rewarded by receiving God's grace. When you commit minor sins, you can do penance for it by doing some good deeds. This is really the same idea as karma.
    Really, the only difference between the Christian concept of sin and punishment and the Buddhist and Hindu idea of karma, is that the Buddhists and Hindus believe that karmic debt that is not resolved in your current life, will have to be resolved in a future life when you are reincarnated, while Christians believe that if the die in sin, they will go to hell or purgatory depending on the seriousness of the sins.