All of the many teachings of the Buddha center on the Four Noble Truths just as the rim and spokes of a wheel center on the hub. They are called 'Four' because there are four of them. They are called 'Noble' because they ennoble one who understands them and they are called 'Truths' because, corresponding with reality, they are true.
The First Noble Truth is that life is suffering. To live, you must suffer. It is impossible to live without experiencing some kind of suffering. We have to endure physical suffering like sickness, injury, tiredness, old age and eventually death and we have to endure psychological suffering like loneliness, frustrations, fear, embarrassment, disappointment, anger, etc.
The Second Noble Truth is that all suffering is caused by craving. When we look at psychological suffering, it is easy to see how it is caused by craving. When we want something but are unable to get it, we feel frustrated. When we expect someone to live up to our expectation and they do not, we feel let down and disappointed. When we want others to like us and they don't, we feel hurt. Even when we want something and are able to get it, this does not often lead to happiness either because it is not long before we feel bored with that thing, lose interest in it and commence to want something else. Put simply, the Second Noble Truth says that getting what you want does not guarantee happiness. Rather than constantly struggling to get what you want, try to modify your wanting. Wanting deprives us of contentment and happiness.
The Third Noble Truth is that suffering can be overcome and happiness attained. This is perhaps the most important of the Four Noble Truths because in it the Buddha reassures us that true happiness and contentment are possible. When we give up useless craving and learn to live each day at a time, enjoying without restless wanting the experiences that life offers us, patiently enduring the problems that life involves without fear, hatred and anger, then we become happy and free. Then, and only then, do we being to live fully. Because we are no longer obsessed with satisfying our own selfish wants, we find we have so much time to help others fulfil their needs. This state is called Nirvana. We are free from all psychological suffering as well. This is called Final Nirvana.
It is a dimension transcending time and space and thus is difficult to talk about or even to think about. Words and thoughts being only suited to describe the time-space dimension. But because Nirvana is beyond time, there is no movement and so no aging or dying. Thus Nirvana is eternal. Because it is beyond space, there is no causation, no boundary, no concept of self and not-self and thus Nirvana is infinite. The Buddha also assures us that Nirvana is an experience of profound happiness. He says: "Nirvana is the highest happiness" Dp 204
The Fourth Noble Truth is the Path leading to the overcoming of suffering. This path is called the Noble Eightfold Path and consists of Perfect Understanding, Perfect Thought, Perfect Speech, Perfect Action, Perfect Livelihood, Perfect Effort, Perfect Mindfulness and Perfect Concentration. Being a Buddhist practice consists of practicing these eight things until they become more complete. You will notice that the steps on the Noble Eightfold Path cover every aspect of life: the intellectual, the ethical, the social and economic and the psychological and therefore contain everything a person needs to lead a good life and to develop spiritually.