Where Schopenhauer saw life as driven by a blind will to exist, Nietzsche sees something more specific: a will to discharge strength, to expand, to surpass. Life sacrifices itself for power, not in the violent sense but in the sense of growth, expression, and self-overcoming. Even the weakest creatures express this drive in their own way.
The Will to Power is not simply a theory of motivation but a principle for revaluation. Every moral system, every table of values, expresses the will to power of those who created it. The noble values of ancient Greece reflected the will to power of an ascending life; the slave morality of resentment reflects the will to power of those who cannot otherwise assert themselves.
The highest expression of the Will to Power is not conquest but self-overcoming: mastery of one's own drives and instincts, the creation of new values from one's own depths. Zarathustra declares it the inner law of life: that even Life itself must continually surpass what it has become.
Will to Power is central to Nietzsche's mature philosophy. His unfinished notes towards a book of that name were assembled posthumously by his sister in a form heavily distorted from his own intentions.