Karma is a very misunderstood and misused term.
It is nothing to do with mysticism, nothing to do with past lives, unless that fits into your world view - there is no _need_ to believe in reincarnation for Karma to make sense.
Karma in the East is Causality in the West. Plain, simple cause and effect.
The reason it starts to take on a mystical character in our minds is due to the scale of the beast contemplated. When chains of cause and effect are linked together and traced back it can be quite overwhelming, not to say humbling.
Generally, in our humdrum, mundane existences, we think of cause and effect in isolation. one shot deals. I let go of the plate, it smashes on the floor.
But, if you think about, well, anything, any single thing, action, whatever. Start thinking about it's lines of cause and effect - that anything you see or do now is an 'effect' which has causes, and those causes themselves are 'effects' which have causes, and so on - and pretty soon, you'll start to see that is absolutely everything (or, if you like, everyone) in this universe (world) is dependent on everything (everyone) else.
Do it.
When you have, is it hard to say that there is an incredible weight of Karma pushing you and your life around? When you see it, you can change your part in it and try to cut a lot of the bad Karma out.
Bad Karma, like... oooh, ummm....
ustwo.
