Thales is remembered for arguing that water is the essence of all things. This argument is significant because it is the first attempt to explain the physical world without reference to a supernatural power. Prior to Thales all such explanations relied on gods or other mythological forces.
Thales lived in the city of Miletus, in Ionia. The Ionians were well-traveled and had many dealings with Egypt and Babylon, and it is possible that Thales had studied in Egypt as a young man. In any event, Thales was almost certainly exposed to Egyptian mythology, astronomy, and mathematics, as well as other traditions alien to the Homeric traditions of Greece. It is perhaps because of this that his inquiries into the nature of things took him beyond traditional mythology.
Thales had a profound influence on other Greek thinkers and therefore on Western history. Anaximander is sometimes considered to be a pupil of Thales. And it is reported by early sources that one of Anaximander's more famous pupils, Pythagoras, visited Thales as a young man, and was advised to travel to Egypt to further his philosophical and mathematical studies. Many philosophers followed his lead in searching for explanations in nature rather than in the supernatural; others returned to supernatural explanations, but couched in the language of philosophy, rather than myth or religion.
Herodotus reports that Thales was with the Lydians when they fought the Medes, and was able to forecast an eclipse which ended the war. The accuracy of the story is questionable, but it at least shows the sort of reputation Thales earned.