Generally, when a character is subject to an unusual or magical attack, he or she gets a saving throw to avoid or reduce the effect. A saving throw is a 1d20 roll plus a bonus based on the character’s class and level (the character’s base save bonus) and an ability modifier.
A natural 1 (the d20 comes up 1) on a saving throw is always a failure. A natural 20 (the d20 comes up 20) is always a success.
A character’s saving throw bonus is: Base save bonus + ability modifier
The Difficulty Class for a save is determined by the attack itself.
Saving Throw Types
The three different kinds of saving throws are:
Fortitude: These saves measure a character’s ability to stand up to massive physical punishment or attacks against his or her vitality and health such as poison and paralysis. Apply a character’s Constitution modifier to his or her Fortitude saving throws.
Reflex: These saves test a character’s ability to dodge massive attacks such as explosions or car wrecks. (Often, when damage is inevitable, a character gets to make a Reflex save to take only half damage.) Apply the character’s Dexterity modifier to his or her Reflex saving throws.
Will: These saves reflect a character’s resistance to mental influence and domination as well as to many magical effects. Apply the character’s Wisdom modifier to his or her Will saving throws.