Ability

Almost everything a Player Character does is affected by his abilities: Strength (STR), Dexterity (DEX), Constitution (CON), Intelligence (INT), Wisdom (WIS), and Charisma (CHA).

The "comeliness" statistic is used in some versions of tabletop D&D but is not used in DDO. Abilities are also sometimes named attributes or stats.

Level ups
Characters can increase one ability of their choice by +1 at levels: 4, 8, 12, 16, 20, 24 and 28. This can be the same ability or different ones over those levels. These levels ups are valid for Ability requisites for Feats.

Ability modifiers
Each ability has a modifier that is 0 for an ability score of 10 and increases by 1 every 2 ability points above 10 (or decreases by 1 every 2 ability points below 10). This modifier is the number you apply to the die roll when your character tries to do something related to that ability. You also use the modifier with some numbers that aren’t die rolls. A positive modifier is called a bonus, and a negative modifier is called a penalty.

When an ability modifier changes, all attributes associated with that modifier change accordingly. Note that a character does not retroactively get additional skill points for previous levels if they increases their intelligence; however, a character who increases their constitution will get additional hit points.

See the modifier page for a table giving the modifier tied to an ability score.

Non-standard uses
Some feats, enhancements and items let you change the ability score that applies to particular mechanics (usually attacks, damage or saving throws)
 * Non-standard attack and damage mods