I tend to think about karma as something along the lines of "every conscious act has an experienced result - but not every experience is the result of a conscious act." I say this because some Buddhist schools treat everything, but everything, that happens to you as being the consequence of your own past actions, and that doesn't add up for me. That approach actually seems to me to be a sort of "stealth egotism", as if the entire universe revolved around me!
I think everything we do (every choice we make, you could say - including choosing inaction) has an effect, there's no avoiding that fact. In particular, the state of mind with which we do things affects what we (and others) experience from our actions. And to me the practical significance of this is: I can't avoid having an effect in the world, so I might as well choose to have a good effect, and devote time to understanding better what that entails.
But at the same time it seems obvious to me that most of what happens to me is just the consequence of living in a huge and complex universe full of things interacting and being changed by each other - so if I catch a cold, I don't assume it's because I was unkind to someone in a past life! The causes could be any or all of a heady blend of physics, chemistry, biology, psychology, emotion, or spirituality... because in spite of what the movies try to tell us, pretty much nothing happens just for one reason.
