Strength
Modifies the character's damage roll. While this is typically a bonus, it is a penalty at particularly low values.
Adds a bonus to the character's carry capacity.
Determines the maximum weight a character's mainhand weapon can be.
Intelligence
Modifies the character's armor class. While this is typically a bonus, it is a penalty at particularly low values.
Reduces all mana costs.
Increases passive mana point recovery.
Wisdom
Modifies the character's saves. While this is typically a bonus, it is a penalty at particularly low values.
Adds a bonus to all forms of active mana recovery, such as cantrips or Brawler's Renewal.
Grants a bonus to healing effects.
Dexterity
Modifies the character's hit roll. While this is typically a bonus, it is a penalty at particularly low levels.
Reduces all move costs.
Increases the range of weight a character's offhand weapon can be when dual wielding.
Constitution
Grants a chance to resist poisons.
Increases passive hit point recovery.
Increases the character's maximum death total.
Charisma
Modifies the character's prices when purchasing from shops. While this is typically a bonus, it is a penalty at particularly low levels.
Increases the character's chances of a successful reward with sacrifice.
Increase the caster level of spells cast.
Hitroll
Hitroll represents your chance to hit a given target. This chance is reduced by the target's armor class, parrying, and dodging abilities. Factors that increase your hitroll include: dexterity, various skills and spells, weapon quality, and enchantments on items or weapons.
Damroll
Damroll represents various factors that increase the damage you do with every physical blow. Things that increase your damroll include: strength, various skills and spells, weapon quality, and enchantments on items and weapons.
Armor class
Your armor class, visible with the 'SCORE' command, tells you how well-armored you are against the physical and magical dangers of the world. It is a combination of natural intelligence, the protection offered by worn items, and magical enchantments or enhancements. Your AC is also affected by your body position--sleeping characters have worse AC.
The lower (more negative) your AC number is, the more effective your armor will be at reducing damage, as a percentage value, with increasing returns for lower values. Against lesser opponents it may help to avoid being hit altogether. On the other hand, having an AC higher than zero will increase vulnerability to damage.
Saves
Two of your primary defenses against magical attacks and effects lay in two areas: saving throws, and resists.
Saving throws (or 'saves') represent your generic resistance to all types of magic. In general, whenever you are the target of a spell or other magical malediction, your saves are factored in when determining whether you resist the effects. Much like armor class, negative saves are better.
They apply to all damage types. However, they also only apply to spells. In the case of (most) damage spells, they halve the damage (there are notable exceptions, like disintegration).
Damage Reduction
Damage reduction isn't a specific stat, but rather a concept not explicitly shown in score. Generally speaking, it represents a percentage by which damage is reduced (a few things, like durability and fugue, reduce instead by subtraction, making them more useful against lower damages and less against higher damages). It can apply to all or almost all damage (like with sanctuary), or just to certain types.
Resistance
Resistance is the cream of the crop, acting as a sort of combination of saves AND damage reduction. However, unlike normal saves, the saves implied by resistance ONLY affect the specified damtype. The number of saves equals the amount of resistance, so for example if you cast ward of frost and gain 25 resistance to fire, you've just given yourself an additional 25 saves when saving against fire type spells, as well as 25% damage reduction. 25 saves is a lot, so you will save very often against fire. Note that this doesn't mean fire-sphere spells necessarily, it means fire-type spells. To be sure, most fire-sphere spells are also fire-type, but to draw an example from another sphere: searing light is spirit-sphere, but of type light, while spiritwrack is also of spirit-sphere, but of type holy.
Having a negative resistance score will make a target more vulnerable to the corresponding damage type.
Overpercent
Overpercent refers to having more than 100% in a weapon skill. Two classes, swordmaster and earth templar, receive abilities which allow them to attune to a weapon and gain overpercent in it. When using such a weapon, any attack or defense that uses that weapon skill in its calculations receives a bonus. This bonus may affect a wide variety of things to varying degrees, but the effect will always be positive. Effective percent in a weapon skill will never exceed 133%.
Hit points
Run out of em' and you're dead. All classes gain the fast healing skill that improves passive health regeneration. Some classes, like water magi, specialize in the restoration of hit points.
Mana
A resource used to fuel most active abilities and spells. All classes gain access to the meditation ability that improves passive mana regeneration, and some classes gain trance which improves it further. Some classes also gain active mana regeneration, such as cantrips and Brawler's Renewal.
Movement points
A resources consumed when walking from room to room, and fuels some abilities. Walking over difficult terrain consumes more movement points, but this can be mitigated by flying. A few classes gain abilities that improve movement regeneration. If your movement is drained, you cannot walk from room to room.