Player Core
Chapter 7: Spells
Defenses
Source Player Core pg. 300
If a spell allows the target to attempt a saving throw or use their AC to defend against it, the type of defense is listed in the stat block. Any details on the particular results and timing of the save appear in the text unless the entry specifies a basic saving throw, which follows a standard rule. If a spell allows a defense only under certain circumstances or at a certain time, the Defenses entry is omitted, since the text needs to explain it in more detail. Whenever a spell allows a saving throw, it uses the caster's spell DC, and one that allows AC as a defense typically requires a spell attack. More information on how to calculate your spell DC and spell attack modifier appear in the links.
Basic Saving Throws
Source Player Core pg. 300
If a spell's Defenses entry specifies a “basic” saving throw, the spell's potential effects all relate to the damage listed in the spell's description. The target takes no damage on a critical success, half damage on a success, full damage on a failure, or double damage on a critical failure. The rules for basic saving throws are found here.