Pandora's box is an artifact in Greek mythology, taken from the myth of Pandora's creation in Hesiod's Works and Days. The "box" was actually a large jar (πίθος pithos) given to Pandora (Πανδώρα, "all-gifted, all-giving"), which contained all the evils of the world. Pandora opened the jar and all the evils flew out, leaving only "Hope" inside once she had closed it again.
Today the phrase "to open Pandora's box" means to perform an action that may seem small or innocent, but that turns out to have severely detrimental and far-reaching negative consequences.
In classical Greek mythology, Pandora was the first woman on Earth. Zeus ordered Hephaestus to create her. So he did, using water and earth. The gods endowed her with many gifts: Athena clothed her, Aphrodite gave her beauty, Apollo gave her musical ability, and Hermes gave her speech.
According to Hesiod, when Prometheus stole fire from heaven, Zeus took vengeance by presenting Pandora to Prometheus' brother Epimetheus. Pandora opens a jar containing death and many other evils which were released into the world. She hastened to close the container, but the whole contents had escaped except for one thing that lay at the bottom – Elpis (usually translated "Hope", though it could also mean "Expectation").
The original Greek word was 'pithos', which is a large jar, sometimes as large as a small person (Diogenes of Sinope was said to have slept in one). It was used for storage of wine, oil, grain or other provisions, or, ritually, as a container for a human body for burying. In the case of Pandora, this jar may have been made of clay for use as storage as in the usual sense, or of metal, such as bronze, as an unbreakable prison.