Karma literally means action, but a more easy way to understand it is to think of karma as habits and thought patterns. Every action you do and actually every thought you think contributes to the constant shaping and reshaping of your habits and your thought patterns. Your thoughts, emotional reactions, way of speaking, and so on, are all strongly influenced by habituation. In the same way, the more consciously you choose sound thoughts, reactions and words, the more you change your own habits and thought patterns in a constructive way.
Everything you are now is a product of this shaping in your past, i e of your thoughts and actions in your past lives and in your present life until a moment ago, and in the same way, you constantly shape your own future by your choice of continuing (and thus reinforcing) your established patterns and habits, or changing them by working on establishing new and more constructive habits and patterns. This will influence your present life from the next moment on, as well as your future lifes.
Your actions, habits and thought patterns not only shape who you are, but also how others will treat you, which is why it is said that the effects of your actions will always come back like a boomerang. If you're aggressive, people will shy you; if you're generous, people will be generous towards you; if you smile, people will smile at you; if you speak the truth, people will trust you; if you lie, people will mistrust you; if you kill, people will hate you; and so on.
If you for instance lie just once, you might think no one noticed, and thus you didn't have to suffer any negative effects. Then you forget the habituative effects, which will make your lies obvious in the long run, ruining people's trust in you. Often it's better to have the negative effects early on, because it will weaken the process of habituation.
The effects of your habits and thought patterns are especially crucial at the moment of death and rebirth, since your present state of mind will highly influnce the path your consciousness will take or be propelled into. That's why the effects of karma on your next life is often stressed. This doesn't mean it only effects your future rebirths. If you study yourself, you can see already now how your actions effect your habits, and how you can deliberately change your habits and thought patterns if you want to.