The D10 Trait SystemEdit
The cWOD game systems use a core game mechanic based around the use of multiple 10-sided dice (d10s). Whenever a character is about to take an action with a reasonable chance of failure, the player makes an action roll using a dice pool. The number of d10s in the dice pool depend upon how many dots the character has in one or more Traits. Traits represent some feature that a character possesses- not all of them measured on a scale. Arete is an example of a Trait unique to mages and the M20 system. Other traits such as Attributes, Abilities, and Willpower are universal to all cWoD systems. For each dot in a Trait about to be used to make an action roll, the player rolls 1d10.
The standard difficulty of M20 is set at 6. That means that the player wants all of those dice in their action pool to be 6 or higher. The more successes that they roll, the more their character can accomplish can accomplish. Both the number of dice in a dice pool or the difficulty pool can be increased or decreased with situational modifiers.
Example: The character Alma Wade has a dexterity of 2 and athletics of 2: she is a woman in her mid-twenties who gets a fair amount of exercise but would never win a marathon. She is running across the street, trying to avoid being hit by a speeding car. This provides a dice pool of 4 for Alma's player to roll. Her degree of success will depend on how many 6s come up.
Alma's player really doesn't want her character to be hit by that car. She wants to give herself a situational modifier by spending a temporary point of Willpower before making her roll to give herself an automatic success. An automatic success ensures that Alma will not botch: rolling at least one "1" and no successes (6 or above). When a character botches, it means that they did not simply fail, their action made everything worse not better. Exactly how much worse is something the Storyteller will decide. If the situation is one where the character might have time to try again, the Storyteller might allow the player to re-roll, increasing the difficulty by 1.
The Storyteller may decide to apply a different kind of modifier. For example, the driver has no desire to hit a pedestrian: the Storyteller might therefore decide that Alma's difficulty is 5 rather than 4. On the other hand, it could be a dark and rainy night, the driver might have a couple of coctails in them or speeding or talking on a cell phone- or all of the above. The Storyteller tells Alma's player that their difficulty for this action roll is 7, 8, or even 9. Alma's player can still spend willpower to obtain their automatic success, but that could be negated by a 1, causing failure. A failed roll isn't good, but its generally less severe than a botch.
A single success means you did an "okay" (marginal) job. Two successes is better, three successess is a completely satisfying result, whereas four successes is an exceptional "better than imagined" result. At five or more successes, the Storyteller should describe an absolutely phenomenal result.