A petrified character has been turned to stone and is considered unconscious. Though a petrified creature has the visual appearance of standing up, it is in fact still susceptible to prone conditions such as knockdown. When the difficulty setting is hardcore or higher, the duration is permanent (for all standard sources of this effect) and the death panel will pop up for petrified player characters.
Petrification is one of the more insidious effects because there is no immunity that protects from petrification itself. The only defenses against this effect pertain to the spell or ability that inflicts it, such as a saving throw or immunity to the spell/ability in question (e.g. immunity to the specific spell flesh to stone). On the other hand, the default scripts that apply petrification do intentionally skip creatures whose flesh cannot be turned to stone, based upon the creature's appearance. This includes creatures without flesh (most golem and skeletal appearances, all elemental and non-corporeal appearances, battle horror, shadow dragon, and animated chest) and creatures that naturally have a petrification ability (medusa, basilisk, gorgon, and cockatrice), but excludes custom content appearances. So, for example, polymorphing into an iron golem (see shapechange) makes one immune to the default petrification.