What is Kabbalah?

What is Kabbalah? How is Kabbalah related to Judaism? Can we say Kabbalah part of Judaism? Or a sect of Judaism?

Kabbalah, popularly understood, refers to Jewish mysticism, especially the school of Jewish mysticism which developed in Spain and South France in the 12th-13th centuries. This school sought to understand and explore the world of the Divine, beyond the grasp of human senses, through meditation and mystical interpretation of holy Jewish texts (the Jewish Bible, Talmud etc).

Kabbalah is part of the accepted tradition of Jewish study of the world and the Divine alongside competing traditions such as rationalist philosophy. While not everyone follows its teachings, it is a part of Judaism and not a sect thereof.