The creature is unconscious.
The creature has resist 20 to all damage.
The creature doesn’t age.
Usually a creature is subjected to this condition when it is turned to stone, such as by a medusa. Often the only way to end this condition is by using a power or a magical ritual.
Normally, a creature falls prone when it becomes unconscious. The DM might decide that a petrified creature instead remains upright, posed like a statue.
Published in Player's Handbook, page(s) 277, Rules Compendium, page(s) 232, Monster Vault: Threats to the Nentir Vale, page(s) 124.