Ars Magica
Let me see if I can explain
Ars Magica in a nutshell. Certainly a challenge.
This is an explanation for D&D 3.x veterans; I'll be using D&D terms instead of ArM terms throughout, only mentioning the ArM terms in parenthesis when appropriate.
Ars Magica (ArM) has much in common with D&D 3e, perhaps due to the shared influence of Jonathan Tweet in their design. But they are also very different.
ArM's core mechanic is the same as D&D: roll a dice, add modifiers, if you beat the DC ("Ease Factor" or "EF" in ArM lingo) you succeed and the amount you surpass it is the degree of success.
Unlike D&D, ArM uses only one dice: a d10. It can be rolled as is, but most things are based on the Stress roll. This is an "exploding" roll, allowing for both very good results and very bad results on rare occasions.
In case you are interested, here is the exact mechanic. If you roll a "1" you pick up the dice and roll again, doubling the result. (If you again roll a "1" you do this again, and so on - for example a result of "1,1,6" means 24). On the other hand, if you rolled a "0" [except on a reroll], you must roll a number of Botch Dice set by the DM ("Storyguide" or "SG" in ArM speak); if some of these come out "0", you botched and failed to do whatever you were trying to do.
While ArM uses virtually the same Ability Scores (which it calls "Characteristics"), it forgoes the 3-18 scale and uses instead simply the modifier. A character can have scores ranging from -5 to +5.
The first major departure from D&D is that ArM is skill-based. You gain experience directly in skills ("Abilities" in ArM terms), which raises their rank ("level" in ArM). Any character can pick up any skill with sufficient in-character justification, with a few exceptions. Like in D&D the skill's rank range is about the same as the dice (ranging from 0 to about 10), but note that the ability scores (characteristics) have more impact as the numbers are lower.
In sharp contrast to D&D XP is not usually gained from adventures, instead being gained through long periods of practice, training, and so on - it actually takes time, patience, and dedication to improve your skills. There is a simple system in place, based on dividing each year into four Seasons, to determine just how much XP you gain in which skill (Ability), or what other benefit your character gained.
Learning or creating new spells are just two examples of things you can do in a Season in addition to gaining XP in a skill (Ability). Other examples include creating or improving magic items, binding or tempering the bond of a familiar, enchanting a talisman that acts as a personal magic item attuned to you, brewing a longevity potion to prolong your life, and more.
ArM campaigns ("sagas") can unfold over centuries, with wizards being nigh immortal and the mundane characters withering and dying to be replaced by other, younger, characters.
There are no non-human races in ArM, but all characters choose a package of traits that seperate them from the others, called Virtues and Flaws. These range from being particularly good with wielding certain types of weapons, to being a lycanthrope. They include such things as faerie-blood if you must play an elf. There are specific rules to govern how to choose this package, but generally you must choose flaws to "balance" your virtues.
The key selling point of ArM may very well be its magic system. It is based on combining a Technique skill that acts as a verb with a Form skill that acts as a noun, to form a sentence of sorts. For example, to create a fireball you would need roll Create+Fire+die+Constitution ("Creo technique+Ignem form+stress die+Stamina" in ArM speak) and beat the DC (EF). Guidelines for judging DCs, and descriptions of the five techniques and ten froms, are given.
ArM wizards can try to cast any spell spontanously, but also know certain ("Formulaic") spells much like a sorcerer has a Spells Known list. These are easier to cast, encouraging players to stick to known and familiar spells while still allowing attempting to do anything with magic.
Spellcasting is a tiresome, physically demanding activity - hence the Constitution ("Stamina") modifier. Failure to cast a known spell, or attempting to cast a powerful spontanous spell, causes fatigue which limits how many spells the wizard can throw around.
Combat in ArM is based on combat skills such as Great Weapon or Bows. The attacker's skill check is compared with the defender's skill check, both modified by Attack and Defense modifiers for the weapon. If the attack overcomes the defense, the difference is added to the attacker's weapon Damage bonus, and the defender's Soak score (based on armor and constitution ("Stamina")) is deducted. The result is applied towards creating wounds.
So combat is based on opposed skill checks, armor serves as damage reduction, and instead of hit points characters suffer a "heavy wound", "light wound", and so on - incurring penalties to their further actions.
There is no instant healing, at least not without spending rare resources, so the characters tend to heal using the Heal skill ("Chirurgy or Medicine Ability").
There are other things that make ArM unique. It's pseudo-medieval setting with its rich political landscape of scheming wizards, it's troupe-style play where every player plays several characters and often switches between different adventures, the wizard's need to collect and trade for rare magical resources and tomes, and so on. But mechanically, I think that's the gist of it.
So, how did I do?
