In the fourteenth century, the Zohar was discovered in Spain, and this book became the basic kabbalistic text.
Although not at all organized in any systematic way, the Zohar still has a fairly clear system of its own, and much of the work of later generations has been nothing more than clarification, expansion, and development of the system.
Altogether, the literature of the Kabbalah is vast, including thousands of books, but the fundamentals, to a degree, can be summarized briefly.
According to the Kabbalah, God acts on the world and reveals Himself through ten aspects, or emanations, called the Ten Sefirot.
These sefirot are the instruments through which the Divine fullness is revealed, God Himself being infinite and devoid of all limits and attributes in the world.
The mutual relationship between these sefirot and their various combinations determine the essential manner and working of the world, and especially of men.
More particularly, the people of Israel react to the union or separation or constellation of the various sefirot, with all their power for good and for ill.
The evil in the world is derived from a distortion of certain forces, and they can, in turn, have a bad effect on the rest of creation.
The Torah, or Jewish Scriptures, is, on the whole, a revelation of the right way to behave so that the Divine plenty will flow into the reality of the world.
The carrying out of the commandments (mitzvot) of the Torah acts in a concrete way to make the sefirot combine properly to cause this plenty to flow, while the transgression of the commandments is an act of absolute evil that adds strength to the forces of wickedness and pollution in the world.
The esoteric teaching, the Kabbalah, is the inner part of the Torah that explains the metaphysical significance of every single movement and thought, and ultimately of the whole essence of the world.
The man who attains genuine knowledge of the wisdom of the Kabbalah can, in certain respects, use the keys provided by this wisdom to reach a deeper and more complete closeness to God, and is able to change and "repair" the world in which he lives.
–Rabbi Adin Steinsaltz
From “The Ari,” in The Strife of the Spirit by Rabbi Adin Steinsaltz