The following are the Four Noble Truths, which the Buddha explained in his first teaching after his enlightenment. These are the fundamental tenets of Buddhism.
First Noble Truth: There is suffering
Second Noble Truth: Suffering is caused by desire
Third Noble Truth: Suffering can end completely
Fourth Noble Truth: The Noble Eightfold Path is the answer to suffering
What do they mean? In a simple sense, the meaning of the First Noble Truth is apparent. We see suffering everyday, in ourselves and others. The First Noble Truth states that desire, selfish craving, and wishing things to be different from how they are is the cause of all suffering. The Third Noble Truth states that the constant state of suffering we experience can totally cease by way of observing the Fourth Noble Truth; following the Noble Eightfold Path.
Possible misconceptions: I have heard the First Noble Truth stated as (to paraphrase) 'All the world and all of life is suffering'. This would lead the casual observer to think that Buddhism has a very pessimistic worldview. Not so. First of all, the word which is translated into the English word 'suffering' is 'dukkha', which has a complex definition and therefore many meanings. For simplicity, I always say that dukkha refers to the inherent 'unsatisfactoriness' of all things, and can represent suffering ranging from slight discomfort to extreme misery.
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May all beings be happy (and free from dukkha)!