Karma is when you believe that your actions now affect your life later on. This is a little different from karma in a religious/spiritual context, the meaning here is just referring to the colloquial uses that I have experienced in which it is used very loosely.
For example, if you are nice to someone today, then tomorrow someone might be nice to you.If you are nice then good things will happen for/to you.
But the opposite will happen as well. If you are mean, bad things might happen to you. The expression "karma's a bitch" is generally used when you did something that wasn't especially good, and it comes back to you. It's kind of a "get what you give" type of thing.