I have three different understandings of "karma"
One is that actions have consequences: if you do good good stuff will happen to you in return, if you do bad..etc. On a large scale I think of this when I consider rural communities where neighbors help neighbors because everyone knows that they are going to need their neighbor's help sooner or later.
The other is that: I deserve everything that happens to me, because it's the will of the universe. It's very similar to the western concept of fate, or the saying "God has plans for everyone". It's a way of thinking that there's good to come out of most events that happen in life, even if the benefits may not be immediately apparent.
But put the two above ideas together, and it seems like I believe fate but I also believe in free will, making it ambiguous as to if I believe fate or if I believe free will. I think I justify both ideas by considering that the situations we encounter where we must make choices are fate but the choice one makes is free will.
The last one involves reincarnation and the cycle of life. I'm not sure where I got this concept from, but I have this notion that if you do many good things during your lifetime, the next time you're reborn you may be rewarded by being born as a higher life form, and if you do bad things, you may be demoted to a lower life form.
The way I understand this is that a dog that acts with virtue (even if it may not understand hat virtue is, it can demonstrate this by, for example, rescuing a toddler from a burning house) will be rewarded by being reborn into a bear or a human or some other "higher" life form. But a husband who neglects his family and gambles all his money away might be reborn as a worm or a dung bettle (not that worms or dung bettles are awful creatures. They are magnificent in their own ways) because they have abused their privilege of being a human.
Since there is reincarnation involved in this idea, I'm assuming it's some kind of bastardization of concepts from the hindi religion.