Oracles are human beings who make predictions, or offer insight, based on a (claimed) connection to the Gods. In the ancient world many sites gained a reputation for the dispensing of oracular wisdom: they too became known as "oracles".
In classical Greece, the pre-eminent oracle was the one at the temple of Apollo at Delphi: Sybil (or Pythia). This oracle exerted considerable influence across the country, and was consulted before all major undertakings -- wars, the founding of colonies, and so forth. She also was respected by the semi-Hellenic countries around the Greek world, such as Macedonia, Lydia, Caria, and even Egypt. Croesus of Lydia consulted Delphi before attacking Persia, and according to Herodotus received the answer "if you do, you will destroy a great empire." Croesus found the response favorable and attacked, and was utterly overthrown.
The oracle is also said to have proclaimed Socrates the wisest man in Greece, to which Socrates said that if so, this was because he alone was aware of his own ignorance. In the 3rd century A.D., the oracle (perhaps bribed) declared that the god would no longer speak there.
Another oracle of note lay in Egypt, in a temple dedicated to Ammon, whom the Greeks associated with Zeus. Alexander the Great had visited it, and though what he asked it is unknown, it is certain the oracle hailed him as Ammon's son, which influenced his conceptions of his own divinity.