A Save is a character's last ditch effort to avoid some sort of peril. Saves don't actually represent skill. They are representations of straight up carnal instinct.
A character's Fortitude Save represents his body's ability to resist death despite overwhelming odds. A Fortitude Save is used in instances when a character's physical being is under duress. This includes Poison, Disease, and many other effects.
Save: d20 + Base Save + Endurance
The Reflex Save depicts a character's subconscious reaction time. The reflex save is used to grab onto a ledge as the character falls, to dodge a blast of flame, or to catch oneself before falling into a pit trap.
Save: d20 + Base Save + Agility
The Will Save represents a character's subconscious armor. It allows characters to resist mental attacks or mental prying.
Save: d20 + Base Save + Willpower
The Presage Save represents the subconscious picking out and avoiding dangers that the conscious part of the mind is unaware of. Its uses include disbelieving illusions, noticing a mental attack, and avoiding being discovered by a mental probe. The result of a successful Presage Save is always a feeling of base unease and suspicion.
Save: d20 + Base Save + Intuition