The desire to predict the future and/or interpret the meaning of present influences has existed since the dawn of history, when cave dwellers first began divining shadows on walls to make intuitive guesses on the outcomes of the next big hunt. Over time, methods became more sophisticated and included everything from the patterns of nature (Augury) to reading entrails (Haruspicy). And although ours is a technological age of logic and analysis, to the Occult Practioner, the utilization of Oracles as a source of wisdom and divining has never lost its efficacy or influence.
In layman's terms, an Oracle is generally a tool used with the specific intent of gaining insight into a situation and the energies present that determine its outcome. Types of things that have been used as Oracles over time include bones, stones, cards, organs, books, smoke, water, fire and the wings of birds.
In short, if it exists, someone has probably tried to make determinations into their future with it.
Although largely speculative, Consciousness Psychologist Julian Jaynes divided aspects of the practice of divination into four basic categories:
Sortilege - Casting lots, sticks, bones or the like.
Augury - Divination that has a subjective interpretation such as dowsing.
Omens - Phenomena or supernal events that are interpreted to determine meaning and impact.
Spontaneous - Channeling wisdom through non-specific avenues such as books or conversation. Cledonism falls within this category.
This provides at least a foundational methodology by which a basic understanding of the practice of divination can be more scholastically studied to some degree.
As meaning can come from any preternatural event or source, the methods of divination are literally as numerous as the things that fill the earth. Some of the more interesting and obsequious forms include:
Complete List of Methods of Divination