Though soothsayer literally means "teller of truth," it refers to a person who predicts the future through spiritual or supernatural means. Often, a soothsayer practices divination, interpreting omens in the natural world or reading messages in specific objects, such as cards or casting lots. A soothsayer may also be a religious figure who receives knowledge from a supernatural source, such as a prophet, oracle, or shaman. The soothsayer has been around for thousands of years, and probably since shortly after the dawn of man, since people are naturally curious about the future and often try to use predictions to plan present actions. Soothsayers are also an important part of nearly every religion.
A soothsayer who practices divination has a number of options open to him or her. Some practices are commonly considered fortune-telling today, though they were taken very seriously in the ancient world and still are by some practitioners. Examples include crystallomancy or scrying, reading a crystal ball; chiromancy or palmistry, reading the lines on the palm of the hand; cartomancy, reading cards, including tarot cards; geomancy, reading markings in the dirt; astrology, reading the stars; and cleromancy or sortilege, casting bones, sticks, or other objects. Some forms of divination that were important in the ancient world, particularly the Greek and Roman empires, are no longer common. For example, haruspicy or extispicy sought messages in the entrails of sacrificed animals, while augury interpreted the flight of birds.
A soothsayer need not use anything external to make his or her predictions. Often, a soothsayer is thought to be inspired or informed by a divine entity. This is the case with biblical prophets, who act as the messengers of God and tell their followers details about the future. In many cultures, notably ancient Greece, the oracle is the source of supernatural wisdom. The Oracle at Delphi is perhaps the most famous of these, and men would travel to consult its divinely inspired priestesses, bringing monetary gifts and animal sacrifices, before any major undertaking. Many native cultures worldwide have traditional healers and soothsayers known as shamans.
As you can see, soothsayer is a rather broad term, but all types of soothsayer serve a similar purpose for the people who believe in them. Often, people place great faith in the soothsayers of their own cultural or belief system and dismiss those of others, but the human desire for spiritual guidance revealed in the figure of the soothsayer is common to virtually all cultures.