In Kabbalah, Ohr is the pleasure that emanates from the Creator (Hebrew "light"). If the qualities of the Kli within the creature are complimentary or equivalent to that of the Ohr, the creature will interpret this as pleasure. All of creation, from Atzilut to Olam HaZeh (this world) pursues this sensation without exception. Every action or inaction is because the creature seeks the sensation of Ohr. When a creature does not seek Ohr, it is because it believes the Ohr will be greater by denying it.
Depending on the way in which the Ohr interacts with the Kli, it takes on different qualities and names. Some of these are Ohr Yashar (Four Phases of Direct Light), Ohr Pnimi (Inner light), Ohr Makif (Surrounding light), and Ohr Hassadim (Returning light).
The larger the Kli, the more light a creature can receive. The smallest will to receive exists in the inanimate level of creation. Such creatures desire nothing more than to retain their form. The next highest is the vegetative level of creation. The larger Kli within these creatures now results in them growing, and needing water and nourishment. The greater the Kli, the more egoism is afforded it, allowing life forms on the animate and speaking levels greater movement, language, and so on.
The size of the Kli is the creature's will to receive. Within Olam HaZeh, the Ohr and the Kli are complimentary. The pleasure from receiving Ohr only exists at the point of contact between them, which is why pleasure is transitory within this world. When a creature attains a Masach, this Ohr can be reflected and a "circuit" may be maintained indefinitely. When the creature is brought into equivalence with the Creator through the attainment of a Masach, the will to receive is clothed with the will to bestow. This mechanism is explained in the parable The Guest and the Host.
Kabbalah is the study of Ohr, the study of pleasure, and hence the study of the law of Creation.