Hermes Trismegistus, or “Thrice Great Hermes”, originated as a syncretic deity in Hellenistic Greece and Egypt. It combined Hermes, the Greek deity most associated with knowledge and magic, with the the Egyptian deity known as Thoth. Thoth is an ibis headed being who was said to have invented writing, astonomy, mathematics, etc. and was believed to serve as the tongue and eyes of Ra. The Greeks identified the similarities between the two and their combined form was worshiped primarily in the city of Khmun, in what is now the Minya province, which came to be known as Hermopolis.
Early Christians had a very different conception of Hermes Trismegistus, however, and figures like St. Augustine thought of him as a human prophet who foresaw the coming of Christianity and was one in a long line of earlier, successive prophets that included Zoroaster and Mani. He has been variously identified throughout the Middle Ages and the Renaissance as a king, a scientist, a supernatural being, a son of a god, a number of different but related people, and many other things.
There are a number of writings, collectively called the Hermetica, attributed to Hermes Trismegistus, the earliest of which is the Smaragdine Tablet which is a short work regarding transmuatation and of great influence to later alchemical practice. The earliest extant source of it is dated to the twelfth century and was written in Arabic. This and many other texts attributed to Trismegistus of obscure origins were complied in a number of forms and translations throughout the Renaissance, when the concepts in them began to take hold. They laid the basis for many occult beliefs and discuss such subjects as the animation of objects, the use of herbs and gems, fortune telling, and countless other practices important to modern New Age beliefs. Another book of note is the Kybalion, which was published in 1912 by three people claiming to be privy to anceint occult secrets.
Two relatively modern organizations heavily involved in Hermeticism and essential to the establishment of its importance are the Rosicrucians and the Hermetic Order of the Golden Dawn. The Rosicrucians are dated to the 15th century by most scholars and combined Hermetic texts with Christianity and other mystical traditions such as Kabbalah. In the 17th century they claimed the coming of a “Rosicrucian Enlightenment” that would be characterized by the use of the secret knowledge of Hermes Trismegistus and the end of armed conflict. There are a number of modern groups claiming continuous descent from this order, although the connection has not been satisfactorily established.
The second organization, The Golden Dawn, was founded in the late 19th century by three Freemasons, one of whom was also a member of the Theosophical society. Known members of this group include William Butler Yeats, Bram Stoker, and Aleister Crowley. They also focused on combining many occult practices and has also been the subject of a number of revivalist societies.
The true identity of Hermes Trismegistus, whether or not he/it ever existed, may never be known. But the influence of the ideas attributed to him to modern occult practice is immeasurable and there are very few New Age traditions that do not owe a great debt to the Hermetic system.