Saving Throws are used to evade some immediate danger or avoid some perilous hazard. If you need to know whether or not the scorpion’s poison overwhelms you, or if you can dive away from a Fireball spell, you make a saving throw. The GM will tell you when you should be making one.
In general, Saving Throws apply as reactions to events that are happening, and are not used to overcome obstacles that need patience or applied knowledge.
In order to succeed at a Saving Throw, the player rolls 1d20 and adds their character's relevant Ability.
Strength for muscle or power saves.
Dexterity for speed, reflexive, or agility saves.
Constitution for resistance or endurance saves.
Intelligence for thinking fast saves.
Wisdom for willpower or concentration saves.
Charisma for personality or charm saves.
Rolling 20 or above indicates success and that the danger is avoided or the action goes as planned.
When failure happens, the GM will narrate the outcome of the failed Saving Throw describing how the characters are affected.
A GM never rolls dice to resolve NPC or monster actions, or negative elements of the environment such as traps - if they involve a PC, the player rolls. Otherwise the GM will make a swift and fair judgement call that moves the story forward and abides by the logic of the unfolding fiction.
Skill checks determine whether your character succeeds at a field of expertise. Failure at the roll means that your character either botched it outright, succeeded in a way that was unhelpful, or was foiled by some unexpected outside influence. The GM will describe the results of failure.
To make a skill check, the player rolls 2d6 and adds his character’s relevant Skill level and Ability modifier. If the total equals or exceeds 10, the check is a success.
Awkward circumstances or bad tools might apply penalties, though usually not more than -1 or -2. By the same token, exceptionally good equipment or a favorable situation might grant bonuses of up to +2 to the roll, or even more if the stars align perfectly. If you lack even level 0 in the relevant skill, you suffer a -2 penalty to your roll.
The relevant skill and attribute modifier will usually be obvious in the situation; attempting to bluff a lone bandit would involve Deception and Charisma, while trying to roll underneath the falling bars of a portcullis would rely on Acrobatics and Dexterity. When in doubt, the GM will tell you what to apply.
If a character needs to overcome an obstacle that isn't covered by a Skill check or a Saving Throw, then the GM will ask for an Ability Check. This works exactly like a Skill check, but the character only adds a relevant Ability modifier, decided by the GM based on circumstances.
There is a gradual level of difficulty associated with each of these checks.
Saving Throws are easy to make, most players are going to succeed at these more than 50% of the time. Saving Throws are always reactions, and failure always leads to injury or harm.
Skill checks are a little more difficult, they require the character to be a little specialized in an area before they can be attempted and most skill checks start with less than 50% chance of success. However, a truly specialized character with a +3 ability modifier and a maximized skill level will succeed at a skill check 99% of the time. Skill checks are always actions.
Ability checks are the most difficult as they almost always only succeed with less than 50% chance of success. Ability checks can be either reactions or actions, but as a rule of thumb failure does not injure or harm the character.
The Saving Throw mechanic is taken from the Black Hack, and skill checks and skill levels were inspired by Stars Without Number.