Why Delphi is special
Delphi in Greece was once the home of an oracle (which defines a shrine dedicated to consulting the prophecies of a deity or the person who acts as the go-between).
Delphi's oracle was the most famous of them all. Common folk and kings from distant lands traveled to Delphi to have their questions answered by the Greek god Apollo.

Questions & answers
Visitors posed queries to Apollo ranging from the personal ("Should I plant more olive trees?") to serious matters of state.
Apollo's answer was heard by a designated priestess who incoherently mumbled it out loud. Delphi priests would then translate the prophecy for the eagerly waiting pilgrim.
The answer (oracle) was often purposely ambiguous to minimize the chances of it being wrong. For example, the reply to a king's "Should my country battle the Persians?" query was "A great kingdom will be destroyed." He interpreted this to mean the Persian kingdom. It turned out it was his that was destroyed.

Impressive site
The site is extensive and lies on a mountain slope with an arresting view of a valley and, in the distance, the Gulf of Corinth.

Over 2000 years in obscurity
The Oracle of Delphi was shut down in 390 AD by the Romans. It wasn't until the 1930s when archaeologists excavated Delphi to its current viewable ruins state.

Today’s major attractions
Delphi highlights include the sacred Sanctuary of Apollo complex, the acoustically near-perfect Theater, and the notable Charioteer bronze sculpture in the Museum.

What leading travel books say about Delphi
Delphi is still steeped in mystery.
Europe
Michelin Green Guides

Delphi has a long and glorious history as the scene of Apollo's famous oracle.
Europe
Frommer's