The Three Lakshanas
The Buddha taught that life has 3 characteristics (the Three Marks of Existence/Three Universal Truths/Three Lakshanas. The Three Lakshanas are anicca, dukkha and anatta. They allow one to see the true nature of reality, and if one does not see things as they really are, this causes them to suffer.
Dukkha (suffering) is the human condition. It is often translated as 'unsatisfactoriness'. Dukkha is the first of the Four Noble Truths. It means that no happiness is permanent and all happiness is tainted by dukkha. Dukkha is a fundamental philosophical concept which requires a lot of explanation.
Anicca (impermanence) means that there is a chain of cause and effect that everything is subject to. An example of the transcience of all things is a flower, which changes and decays. Paticcasamuppada means that everything has a conditioned existence. Everything is interconnected. This relates to Heraclitus' theory of flux - 'you cannot step in the same river twice'.
Anatta (non-self) is the view that nothing about the person exists eternally without change. The self is dependent on 5 skhandhas: body, sensations, perceptions, mental formations and consciousness. There is no eternal soul after rebirth. The flame and rope analogies can be used to describe the process of death and rebirth.