Generally, when you are subject to an unusual or magical attack, you get a Saving Throw to avoid or reduce the effect. Like an attack roll, a Saving Throw is a d20 roll plus a bonus based on your class and level (see Classes), and an associated Ability Score. Your Saving Throw modifier is:

Base save bonus + ability modifier

Saving Throw Types: The three different kinds of Saving Throws are Fortitude, Reflex, and Will:

Fortitude: These saves measure your ability to stand up to physical punishment or attacks against your vitality and health. Apply your Constitution modifier to your Fortitude Saving Throws.

Reflex: These saves test your ability to dodge area attacks and unexpected situations. Apply your Dexterity modifier to your Reflex Saving Throws.

Will: These saves reflect your Resistance to mental influence as well as many magical effects. Apply your Wisdom modifier to your Will Saving Throws.

Saving Throw Difficulty Class: The DC for a save is determined by the attack itself.

Automatic Failures and Successes: A natural 1 (the d20 comes up 1) on a Saving Throw is always a failure (and may cause Damage to exposed items; see Items Surviving after a Saving Throw). A natural 20 (the d20 comes up 20) is always a success.