Everything is interconnected. Everything affects everything else. Everything that is, is because other things are. This is the teaching of Dependent Origination.
This teaching has many names. It is called Interdependent Origination, or (Inter)dependent Arising, or Co-Arising, or variations thereof. It's called Conditioned Genesis and Causal Nexus and many other things. It would be nice if English-speaking Buddhists could agree on a term, but so far we haven't. The Sanskrit is Pratitya-samutpada. I've found the Pali name spelled Panicca-samuppada, Paticca-samuppada and Patichcha-samuppada.
Whatever it is called, Dependent Origination is a core teaching of all schools of Buddhism.
Nothing Is Absolute
No beings or phenomena exist independently of other beings and phenomena. All beings and phenomena are caused to exist by other beings and phenomena. Further, the beings and phenomena thus caused to exist cause other beings and phenomena to exist. Things and beings perpetually arise and perpetually cease because other things and beings perpetually arise and perpetually cease. All this arising and being and ceasing go on in one vast field or nexus of beingness. And there we are.
In Buddhism, there is no teaching of a First Cause. How all this arising and ceasing began, or even if it had a beginning, is not explained. The Buddha emphasized understanding the nature of things as-they-are over speculation of what might have happened in the past or what might happen in the future. It might be said that the Buddhist version of Genesis is: Stuff happens, because other stuff happens.
Also, things are the way they are because they are conditioned by other things. You are conditioned by other people and phenomena. Other people and phenomena are conditioned by you.
The Buddha explained,
When this is, that is.
This arising, that arises.
When this is not, that is not.
This ceasing, that ceases.
Nothing Is Permanent
Dependent Origination relates to the doctrine of Anatman. According to this doctrine, there is no "self" in the sense of a permanent, integral, autonomous being within an individual existence. What we think of as our self, our personality and ego, are temporary creations of the skandhas -- form, sensation, perception, mental formation and consciousness.
So there you are, an assembly of phenomena generating the idea that there's a permanent "you" separate and distinct from everything else. These phenomena (form, sensation, etc.) were caused to arise and assemble in a certain way because of other phenomena. These same phenomena are perpetually causing other phenomena to arise. Eventually, they will be caused to cease. Everything in the phenomenal world is dukkha (suffering or unsatisfying), anicca (impermanent) and anatta (without individual essence; egoless).
Put another way, "you" are a phenomenon of the Causal Nexis in much the same way a wave is a phenomenon of ocean. A wave is not a piece of the ocean in the same way a brick is a piece of a wall. A wave is ocean. Although a wave is a distinct phenomenon it cannot be separated from ocean in the way a brick can be taken out of a wall. When conditions cause a wave, nothing is added to ocean. When the activity of wave ceases, nothing is taken away from ocean.
The Core of Dharma
His Holiness the Dalai Lama said that the teaching of Dependent Origination precludes two possibilities. "One is the possibility that things can arise from nowhere, with no causes and conditions, and the second is that things can arise on account of a transcendent designer or creator. Both these possibilities are negated."
His Holiness also said, "Once we appreciate that fundamental disparity between appearance and reality, we gain a certain insight into the way our emotions work, and how we react to events and objects. Underlying the strong emotional responses we have to situations, we see that there is an assumption that some kind of independently existing reality exists out there. In this way, we develop an insight into the various functions of the mind and the different levels of consciousness within us. We also grow to understand that although certain types of mental or emotional states seem so real, and although objects appear to be so vivid, in reality they are mere illusions. They do not really exist in the way we think they do."
The Twelve Links
There are vast amounts of teachings and commentaries on how Dependent Origination works. The most basic understanding usually begins with the Twelve Links, which are said to describe a chain of causes that lead to other causes. It is important to understand that the links form a circle; there is no first link.
The twelve links are ignorance; volitional formations; consciousness; mind/body; senses and sense objects; the contact between sense organs, sense objects, and consciousness; feelings; craving; attachment; coming to be; birth; and old age and death. The twelve links are illustrated in the outer rim of the Bhavachakra (Wheel of Life).