Karma Ghost is a very cool funny flash cartoon by Billy Bob which gives you a good experiential explanation of Karma.
Usually Karma doesn’t happen this fast or we’d all clue in quickly and start living differently right away. But, on the other hand, if karma wasn’t delayed, we’d have no free will.
Karma can be very complex. But, on one hand, it’s simple. It’s the ultimate accountability. In addition to your inherited karma, you create new karma every day, which creates your future.
Everything you think, say and do creates karmic seeds that, over time, gradually ripen and produce a result in your life. What kind of karmic seeds you plant, determine what kinds of results you get later in your current life and your future lives (if you believe in reincarnation). Karma is a fascinating subject.
Read more on karma from various Buddhist perspectives
Karma from various Vedanta or Hindu perspectives
How people treat you is their karma; how you react is yours.
― Wayne W. Dyer
Once you’ve lived a little you will find that whatever you send out into the world comes back to you in one way or another. It may be today, tomorrow, or years from now, but it happens; usually when you least expect it, usually in a form that’s pretty different from the original. Those coincidental moments that change your life seem random at the time but I don’t think they are. At least that’s how it’s worked out in my life. And I know I’m not the only one.
― Slash. Lead Guitarist of Guns N’ Roses
If you’re really a mean person you’re going to come back as a fly and eat poop.
― Kurt Cobain