Armor Class

Armor Class (AC) is a rating used to determine how difficult it is to damage a creature/character. It is based on several factors such as a creature's natural aversion to physical injury, magical enhancements, and any protective garments worn. The base stat of Dexterity grants bonuses to AC.

* In some editions of the game prior to 3.0, Armor Class ranges from -10 to 10. Having an AC of 10 was the weakest, and a -10 being the strongest possible written AC.
* In 3.0 and 3.5 editions, armor class instead starts at 10 and increases. Extremely non-dextrous or non-moving creatures may suffer penalties that lower their armor class below 10.

Hit points

Hit points (HP) are a measure of a character's vitality or health; they are determined by the character's class (certain occupations breed hardier people) or race, and Constitution score. Hit points are reduced whenever a character takes damage. Typically beings fall unconscious at 0 HP. Living creatures reduced to negative HPs continue to lose additional HPs due to bleeding, etc. unless they are stabilized by chance or healing (natural or magical). At -10 HP a being is officially dead.

Saving throws

Certain situations give characters the chance to avoid special types of danger or attacks. These chances are called saving throws or saves. A saving throw is made when a character would come to harm from extraordinary means such as poisons and magical compulsions in nature.

Pre-d20 System

In the pre-d20 System editions of D&D, there are five categories of saving throws:

* Paralysis, Poison, or Death Magic
* Petrification or Polymorph
* Rods, Staves, Wands (magical devices)
* Spells
* Breath Weapons (such as with dragons or gorgons)

Actions

The combat mechanic is turn-based and operates in rounds. A round is a discrete time interval (approximately 6 seconds, game-time) in which all involved parties act in the combat. The order in which parties involved in the combat act is determined by Initiative.

* In pre-3.0 editions of the game, characters are allowed to move their speed and attack every round, or perform a reasonable combination of other actions.
* In 3.0 and 3.5 editions, what a character can and cannot do in a given round is more codified; a character may perform one full-round action, one standard and one move action, or two move actions in a round, along with any number of free actions, and a single swift or immediate action. Unlike other types of actions, immediate actions may also be taken during someone else's turn, though that counts as using the immediate action slot for the character's following turn.

Skills
Main article: Statistic (role-playing games)#Skills

Dungeons and Dragons, starting with the 2nd Edition of the game and continuing to the current 3rd Edition, has many skills that characters may train in. In the 2nd Edition these were broken down into Weapon and Non-Weapon Proficiencies. In the 3rd they are all simply referred to as "Skills", not to be confused with "Feats", below. Characters gain skill points for buying skill ranks based on class, level, and intelligence. Some skills can only be taken by certain classes, such as Read Lips or Animal Empathy. These skills are called exclusive skills. Others can be used even if the character has no ranks in that skill (i.e., is not trained in that skill).

A skill check is always a d20 roll, with bonuses from the number of skill ranks, the skill's key ability, and any miscellaneous modifiers (from spells or racial abilities, for instance). Sometimes, a skill check may be aided by favorable circumstances (such as you brandishing a weapon while using Intimidate) or hampered by unfavorable circumstances (such as using improvised tools to pick a lock).

An example of a skill is Search, which is Intelligence-based; an example of a miscellaneous modifier which could be applied to search is the +5 competence bonus for a character wearing the "Goggles of Minute Seeing". Other skills include Diplomacy (CHA), Escape Artist (DEX), Swim (STR), various Knowledge skills (like Knowledge (Arcana) or Knowledge (Local)) (INT), Spot (WIS), and Concentration (CON).

A "check" is successful when the roll is higher than or equal to the difficulty class (DC) of the task. Usually, the Dungeon Master sets the DC. Sometimes the DC is set by the result of something else's check, this is an "opposed check". An example of an opposed check is spot against hide: the character is trying to see something else that is hidden/trying not to be seen.

Taken from Wikipedia