Metéora is a formation of immense monolithic pillars and hills-like huge rounded boulders which dominate the local area.
It is also associated with one of the largest and most precipitously built complexes of Eastern Orthodox monasteries in Greece, second in importance only to Mount Athos. The six monasteries are built on natural conglomerate pillars, at the northwestern edge of the Plain of Thessaly near the Pineios river and Pindus Mountains, in central Greece.
Metéora is included on the UNESCO World Heritage List (above info comes from Wikipedia).
For those of you that want to visit this amazing place of unique beauty i need to mention that Meteora are far away from Athens (around 4 hours by train) but close to other cities like Trikala, Karditsa and of course small town of Kalabaka built almost under the rocks.
I would suggest you visit Meteora during autumn or winter as winter colors give a unique beauty to the whole place. In addition skip summer as region is quite warm and you will certainly need a car to reach monasteries. But if you want to feel the vibes of this holy area prefer to walk up to the rocks.
Also in winter fog or light snow, which is common to the place, can make rocky formations scary coming out from films.