The two extremes in which the Buddha speaks of are: that conjoined with the passions and luxury, low, vulgar, common, ignoble, and useless. The second is: that conjoined with self-torture, painful, ignoble, and useless. The avoidance of these two extremes is the path to enlightenment of the middle path as the Buddha had. The middle path produces insight and knowledge, and tends to calm, to higher knowledge, enlightenment, Nirvana.
The Buddha speaks of four noble truths. They are as follows:
1) The noble truth of suffering.
2) The noble truth of cause of suffering.
3) The noble truth of the cessation of suffering, the cessation without a remainder of craving, the abandonment, forsaking, release, non-attachment.
The noble truth of the way leads to the cessation of suffering. The eight-fold path is namely, right views, right intention, right speech, right action, right livelihood, right effort, right mindfulness, and right concentration.
Nirvana is the absolute enlightenment that leads to the purification of the soul. It is achieved through the belief in the four noble truths and the action taken to use or follow them. This all together provides enlightenment and the ultimate goal of reaching Nirvana. Nirvana also means, you are at one with Buddha and have gained the respect of him. Ultimately it is similar to the belief in heaven and God.