Tarotology is the theoretical basis for Tarot Reading, which is a subset of Cartomancy (the act of using cards to scry into the past, current, and future situations). The art itself is hotly debated as to whether or not the cards are guidance from a spiritual force, or that the belief that the cards are instruments used to tap into a collective unconscious or brainstorming subconscious. Deeply woven in the art of tarot is one basic understanding:
"All men had shared a common language, common customs, a common culture and a common religion, which was a reflection of "an eternal and immutable order which unites heaven and earth, the body and soul, the physical and the moral."
The occult Tarot as we know it began like a common deck of playing cards. Four suits, 10 cards numbered from one to ten, with four face cards per suit, for 14 cards per suit. There was also a deck of 21 "trump cards", of which include "The Fool", which would act as the top trump, much like our modern day "Joker".
For most european countries, Tarot cards are still used to play card games. However, in English speaking countries, tarot cards are used for divinatory purposes. Occultists took the trump cards and the fool as the "Major Arcana" (or Greater Secrets), while the suited cards became known as the "Minor Arcana" (lesser secrets). The split is widely regarded to Ancient Egypt or the Kabbalah, but no documented evidence of such origins exist.
Playing cards first entered Europe in the 14th century, most likely brought there from Egypt. Once they reached Italy, additional trump cards were added, and now we find the oldest surviving tarot decks can be traced to mid 15th century.
In the 1783, the cartomancer Jean-Baptiste Alliette devised a method for tarot divination (captured in his treatise of tarot: The Book of Thoth). Known also as "Etteilla", he created the first corrected Tarot, The Grand Ettielle deck, aimed at fixing errors that had resulted from misinterpration and corruption of the cards' meanings through the mists of antiquity.
Published in 1909, Arthur Edward Waite, a Christian mystic, pulished tarot cards under the Rider-Waite deck, which is currently one of the highest published tarot decks in the world. This deck took the occult measures laid out from the time between Etteilla and focused them through the lens of the Hermetic Qabalah, which infused the meanings and images with Qabalistic principles.
Tarot cards have seen a boost in mainstream popularity since, with hundreds of new decks with variations and art being published. If you're interested in buying a tarot deck and don't yet have one, please remember that your first deck should either be a gift, or one that you're incredibly drawn to. It should not be just an impulse buy - be respectful of their ability to tap into the powers we don't understand.