The world and the whole universe is in a constant state of change, a state of flux. Everything that comes into existence, everything that appears, eventually disappears, becomes non-existent. And even while things are in that state of existence, that are constantly changing.
This state of flux can be called impermanence. The term, impermanence, is usually used in the negative sense to indicate the decay and disappearance of phenomena, but it can also mean the impermanence of the featureless - the coming into existence of things that weren't there before.
This can be likened to a pot of boiling water - bubbles form and rise to the surface on convection currents, move around on the surface and sometimes merge with other bubbles, and eventually burst and disappear, only to be replaced with other bubbles. I sometimes visualize the universe as one huge pearl-like planetoid, appearing smooth and flawless from a distance, but seen to actually have a boiling surface of erupting plumes of mass that sink back into its interior when examined closely. Constantly transforming, but with no net change.
Zen Master Dogen said that impermanence can be clearly seen by everybody. "It is not a matter of meditating using some provisional method of contemplation," he says in Shobogenzo Zuimonki. "It is not a matter of fabricating in our heads that which does not really exist. Impermanence is truly the reality right in front of our eyes. We need not wait for some teaching from others, some proof from some passage of scripture, or some principle. Born in the morning and dead in the evening, a person we saw yesterday is no longer here today —these are the facts we see with our eyes and hear with our ears. This is what we see and hear about others. Applying this to our own bodies and thinking of the reality of all things, though we expect to live for seventy or eighty years, we die when we must die."
The Buddha taught that all things rise from conditions. The condition that causes the impermanence of all things is karma. Karma is probably one of the most misunderstood of Buddhist concepts, misunderstood by many non-Buddhists and Buddhists alike. For the purposes of this discussion, it is nothing more than cause-and-effect, the simple observation that everything arises from some condition or set of conditions. Karma is the metaphorical fire than keeps all phenomena boiling in a state of constant impermanence, the hot core of that pearl-like planetoid.
Some people take it further, maintaining that it is also our accumulated good and bad merit (or non-merit), that all somehow eventually balances out in the end. Some maintain it is even carried into the next incarnation, and determines the happy or sad conditions of the next re-birth.
The later concept has a lot of explaining to do. Even if it transcends the dualistic distinction between "good" and "bad," it still gets stuck on the concept of an individual self bearing this lifelong karma, when the Buddha also taught that all things are an interdependent and seamless whole, with no individual self except in our own minds. And it fails to explain the reality right before our eyes - we all know of perfectly awful people who enjoy seemingly endless good fortune, and have all seen chains of tragic events fall on the kindest, seemingly least-deserving people.
I've read Buddhist teaching that try to reconcile these problems with theoretical karmic "seeds" and "fields," but these theories get more and more complicated as they try to resolve one contradiction after the other, and ultimately they are just long speculative theories without one shred of concrete evidence, other than claims that the truth is revealed to those who've managed to enter into very, very deep states of mediation - always deeper than any state you've ever attained, of course, so you just have to take it on faith.
In The God Delusion, Richard Dawkins mentions an Asian woman who tells him not to feel pity for a severely birth-deformed relative, as her misfortune to be born that way is surely a sign that she must have dome something terribly wicked in a formal life. Dawkins rightfully find this lack of compassion despicable, but it is based on what I argue is a misunderstandng of karma and not on the Buddha's teaching, which emphasized no-self and interdependence.
One of the things that's attracted me to Zen is that while it doesn't necessarily renounce this latter concept of karma and of transmigrating souls, it states that worrying about other states of existence and future lives is impractical, when there's so much else to deal with in this life, right here, right now. Since we, like all the rest of the universe, are in a constant state of change, moment to moment, nanosecond by nanosecond, we are constantly being reborn into the present moment, and this continuous reincarnation is affected by our karma of this moment. Unlike some accumulated karma or some karmic seed in a karmic field, when we bear anger, we carry that anger into the next present moments, and when we're practicing kindness or generosity, we carry that into the next present moment.
So karma as nothing more than cause-and-effect is indisputable - no plant exists without moisture or a seed, no thought arises without a mind to conceive it, no deed is performed without someone or something that does it. Everything has its cause. And since it can plainly be seen that the whole universe is in a constant state of flux, everything moving, transforming, entering and leaving existence, this impermanence is fueled by the law of cause-and-effect - karma.
Practically, this means that we have the means, if not of controlling, of at least affecting our continuous rebirth into the next moment. If you want, for whatever reason, to experience the effects of anger or fear from someone, behave aggressively or cruelly toward them. If, on the other hand, you want to experience acceptance and approval, behave with compassion and tolerance. Your cause will determine the effect you encounter.
For the best karmic effects, though, try just sitting still with a quiet mind and seeing what happens with open-minded acceptance.