Thoth (pronounced "Toth", rhyming with "both") was one of the most important deities of ancient Egyptian religion and worship. Typically portrayed with the head of an ibis (a big, sacred wading bird), Thoth was known as the tongue of Ra, who had cursed him, and often spoke on Ra's behalf. Because his job was to speak the words that fulfilled Ra's wishes, Thoth is credited with creating the heavens and earth.
Although he is referenced in some sources as being Ra's son, there is also a theory that Thoth managed to create himself by using the power of magical language. He is known as the creator of magic and the messenger of the gods. Thoth is also referred to in some stories as the keeper of divine records, adviser to the gods, and mediator in disputes. He appears in a few legends as the god who weighs the souls of the dead, although many other stories assign that job to Anubis.
Because Thoth is a lunar deity, he is often portrayed wearing a crescent on his head. He is closely associated with Seshat, a goddess of writing and wisdom, who is known as the scribe of the divine. The Greeks saw him as Hermes, and so the center of Thoth's worship in the classical world was found in Hermopolis.
Thoth appears in a significant role in the legend of Osiris and Isis. When Osiris was murdered and dismembered by his own brother, Set, his lover Isis went to gather up his pieces. It was Thoth who provided her with the magical words to resurrect Osiris so that she could conceive his child, Horus. Later on, when Horus was killed, Thoth appeared to help in his resurrection too.
Thoth enjoyed a bit of a resurgence in popularity when Aleister Crowley published The Book of Thoth, which is a philosophical analysis of the Tarot. Crowley also created a Thoth Tarot deck.
Thoth is sometimes called upon for workings related to wisdom, magic, and fate. He may also be invoked if you're working on anything to do with writing or communications -- creating a Book of Shadows or writing a spell, speaking words of healing or meditation, or mediating a dispute.