Karma-Sanskrit: कर्म; means action, work or deed; it also refers to the principle of causality where intent and actions of an individual influence the future of that individual. Good intent and good deed contribute to good karma and future happiness, while bad intent and bad deed contribute to bad karma and future suffering.
You can interpret Karma in as many ways as there are people, but basically Karma means: "you get what you give", "you reap what you sow" and the ever popular, "what goes around, comes around". This seems simple enough, but ls it? Some people read the explanation of Karma (above) and interpret it as something that will "get back" at someone who has hurt you. It is individual, it is about YOU, not about "getting" someone else. You only need to watch what YOU are doing.
Some people feel that they can do or say whatever is on their mind and not take any notice of anyone else. Surely this will come back to bite them. Right? The consequences of someone else's actions are not your concern nor is it what Karma is about. Karma is NOT about revenge. The way you react to a situation is your Karma, if someone handles the same situation badly that is none of your business, it is their Karma.
Your Karma is about YOU and only YOU. What YOU do/say returns to YOU in kind. You are not responsible, nor can you control, anyone else's Karma. If YOU want to get more, give more. This refers to anything happiness, love, money, anything. YOU GIVE, YOU GET! That's how the universe works.
"How people treat you is their Karma, how you react is yours" Wayne Dyer