I started to write up a detailed post about Saving Throws, but then remembered this post on Blackrazor that does a much better job than I would have: http://bxblackrazor.blogspot.com/2014/09/saving-throws-eh-who-needs-em.html
The definition of Saving Throw could be boiled down to "Luck". It is essentially a luck stat. It could be destiny, fate, serendipity, happenstance, but calling it Luck is the most succinct description. This is one of the reason I like the simplified ST roll in Swords & Wizardry so much. It boils it down to its essence so that each class has its own Luck stat. But...
As so often happens when things are oversimplified it can lead to situations where it doesn't make much sense. Why should a Fighting Man have the same 'luck' against petrification as against a charm spell?
In Warriors of the Red Planet the solution was to go back to the traditional multiple Saving Throw stats, but customized to the genre: Explosions, Mentalism, Energy, Poison, Falls, and General.
Colonial Troopers and Guardians use a different tact. They use the single ST stat, but then let the ability modifiers adjust it for specific circumstances:
Saving Throws Versus Terrible Events
|Strength||Crushing Events, Disarm Events, Knockback Events|
|Intelligence||Memory Events, Puzzle Events,|
|Wisdom||Perception Events, Deception Events, Mental Attack|
|Constitution||Death, Deprivation, Disease, Endurance, Poison,|
|Dexterity||Evasion Events, Balance & Coordination Events, Speed Events|
|Charisma||Ego Events, Emotion Events, Fast Talk Events|
Which system works best? I'm not sure there is an answer to that, or if there is a perfect Saving Throw system. My experience in many games is that it is a great mechanic that facilitates great tension and excitement in the game.