Swami Gulagulaananda said:
"Karma is the greatest philosophy in the world - The only thing that will follow you even after your death"
Karma - One of the fundamental principles in Hinduism. Karma, in the simplest sense, is your account. When you do good things or bad things, it is constantly updated. When you do a bad thing, you are in debt, and when you do a good thing, you get a benefit. In Hinduism, after you die, Chitragupta, Yama's accountant will analyse this account sheet and Yama will pass a judgement as to where you are to belong.
However, when I look around, I feel that because of the population burst, Yama and Chitragupta are running out of space to store all the data of so many people. So, instead of doing server side processing, they have managed to create a system of client side processing - Yes, I mean that most of your deeds, unless extremely critical, are stored in a local cache, and an automatic system will monitor, and when possible take appropriate action to clear out the stored data by taking action.
And if you don't understand the above paragraph, fret not... I am just saying that instead of waiting till death, Yama has devised a technique of giving you rewards, bonuses or punishments right here in this very birth. You see, when you do something bad, whatever it is, you might get coal from Santa Claus, but Yama will ensure that poetic justice is delivered. And thus, as you sow, so you reap.
I have seen many incidents occurring around me to come to this conclusion - A man was too busy to come to his mother's funeral because he had "important" meetings... *Bamm* He lost his job, and had to fight for it for a really long time (and he is still fighting it). A man had loving children who were constantly insulted and humiliated by him. They felt really hurt. *Bamm* Now he is all alone, and he doesn't have a face to go anywhere. A girl loved a guy a lot. He took her for granted. *Bamm* She is not with him, and he repents it every day. And so on...
You see, these might be small incidents, but I don't want to narrate the actual stories because they get too long. The point is that, For every action, there is an equal and opposite reaction. Cause and Effect. As you sow, so you reap. Call it what you want, the result is the same, and that you will get back what you are giving, and many times, with interest. The beauty is that you will definitely get it back - You may not remember it, but it will always come back because of it. If you do good things, people might remember you some day and say "Oh he/she had helped me that day... so let me help him/her today" and if bad, they will definitely not want to miss an opportunity to get back at you.
Thus, always be careful about what you do in life. Be a good person as far as possible, and when you do a bad thing, normally the equity gained from the good will cancel it out. So, be good as much as possible - For Yama Dharma is watching you.
You can NEVER escape Yama's judgement - The Karma will follow you even to your next birth.