It has been said in many oral traditions that everything we think, say and do will ripple forth for the next seven generations. This also applies to those who don't happen to have children of their own. So how does this work, anyway?
We can understand this more easily when we wrap our minds around the fact that we are vibrational transmitters. Our thoughts, our actions and our words are energies that create a ripple in the pond of universal energies. Spirit then feels this and responds. As it responds, what we have sent out comes back to us, magnified several fold. This is also called "instant karma", which is totally a groovy thing because it means we don't even have to send out bad thoughts at the fool that cut us off in traffic, because that same fool will have some of that negative energy he just created bite him in the keester.
But the way it ripples so long and far (the 7 generations thing) is quite amazing. That is where generational karma comes into effect. What our ancestors sowed, we now reap. And we have to make sure that what we sow is something our future generations will appreciate reaping. After all, we are taking care of the earth for them, not for us. Yes, this also goes into the whole tree hugging thing. If we don't take care of the earth mother, then she won't be able to support us. We may not see that happen in our lifetime, but others will in theirs. This is why we need to respect her and care for her. The future generations are only loaning her to us for right now. It is much the same as an inheritance that is put into the care of a custodian until the child comes of age. It can be used to provide for the growing child, but it does not belong to the custodian.
So it becomes very important to watch not just what we say and do, but also what we think, because those thoughts will ripple out as well. And we want positive ripples to return. This does not mean that we don't slip up now and then. But when we do, we need to replace it with a much more positive vibrational thought. It also doesn't hurt to say "cancel that" to the negative one. Spirit will hear that as well. ;-)