Statistics are a measure of your characters power in a particular field. All stats have a use for some characters. Basic cap (from level 1) is 255, plus 5 points for every level above 60, bring us to 280 at level 65, before AAs, and Tomes.
Heroic Power AA raises the cap by 5 per rank with 5 ranks, bringing us to 305. Non-hybrid casters also get the Innate Enlightenment AA, which raises their Intelligence/Wisdom caps by 10 per rank with 5 ranks on top of the Heroic Power boosts, bringing those to a maximum of 355. Any of these values can be considered a visible cap, and your stats will not display as higher than this in game.
The Hidden Strength AA allows some benefit to be gained from stats reaching beyond the visible cap (whatever it is for any particular level and distribution of AAs) by 30 per rank for 6 ranks (although it doesn't do anything for Intelligence/Wisdom), bring stats as high as 485. Hidden Strength-enabled stats count for much less than pre-visible cap stats, however.
Next, there is Tomes for Statistics Enhancement. There is a specific line of tomes for STR, STA, AGI, DEX, and CHA. Each line of tomes has four ranks, and each tome further increases the Hidden Strength cap for that specific stat by 10 points. This means, once complete with all four ranks the hidden cap of that stat will be 525.
Over visible cap statistics for Intelligence and Wisdom now also has a point for all classes due to the Tome of the Mind. This Tome will give the player +1% Healing critical rate for 20 WIS over visible cap and +1% Spell critical rate for 20 INT. The tome has four ranks and each rank will grant the player another crit for an additional 20 points over the previous hidden cap. To clarify: Non-hybrid casters will get +4% critical rate if their Intelligence/Wisdom is 435, and they have completed all four ranks of Tome of the Mind. Every other class would only need 385.
Each character requires a differing set of statistics to accomplish their 'life' missions. Some statistics are more important for one class then another. For instance, warriors require Agility to be a successful tank however a wizard would rather have Intelligence on their gear so they have more mana to cast spells.
The statistics that characters live with are determined by the player upon creation of the character and, afterwards, can only be modified by spells, items and Alternate Advancement points. All abilities use the human model as the base point and the different races are varied from that base to reflect their unique differences from humans.
See also: Spell: Strength
The statistic STR. The Hidden Strength AA makes STR over the cap increase your damage.
From the in-game description: Strength is one factor in determining how much damage you do with weapons and special attacks. It directly affects the amount of weight you can carry before suffering penalties.
See also: Spell: Stamina
Stamina also effects the length of the stamina bar (this bar is yellow and found underneath hitpoint and mana bars in the default UI.) This bar is essentially a timer for many Styles for melee classes.
There are spells available to decrease the stamina used by styles, however there is no way to increase the stamina regeneration rate.
From the in-game description: Stamina increases your maximum hit points. Above 305, if you have hidden strength, stamina also increases your mitigation. Different classes get more hit points per point of stamina than others. Stamina also determines your maximum air supply when you are underwater or in airless zones.
See also: Spell: Agility
The statistic AGI. The Hidden Strength AA makes AGI over the cap increase the chance to avoid.
From the in-game description: Agility is one factor in determining your avoidance--how often foes will miss when trying to hit you. Agility also affects movement speed and defensive skill ups. If your agility is below 75 you will move slower than normal but having agility 75 or higher allows for you to move at a normal pace.
See also: Spell: Dexterity
From the in-game description: Dexterity is a factor in determining how often you will hit foes in melee combat.
Wisdom (WIS) is the primary spell-casting attribute for Paladins, Rangers, Beastlords, Clerics, Shamans, and Druids. It affects how much mana these classes have, which determines how many spells they can cast before they must rest (meditate). Wisdom also determines how often a spell will fizzle for these classes or, in other words, fail to cast.
You receive 12 mana per WIS up to 200 WIS at level 65.
You receive 6 mana per WIS for WIS 201+ until visible cap at level 65.
From the in-game description: Wisdom affects your chances of gaining skill points in most skills, if it is higher than your Intelligence.
Intelligence (INT) is the primary spell-casting attribute for Shadow Knights, Bards, Wizards, Enchanters, Necromancers, and Magicians. It affects how much mana these classes have, which determines how many spells they can cast before they must rest (meditate). For all except Bards, it also determines how often a spell will fizzle or, in other words, fail to cast.
You receive 12 mana per INT up to 200 INT at level 65.
You receive 6 mana per INT for INT 201+ until visible cap at level 65.
From the in-game description: Intelligence affects your chances of gaining skill points in most skills, if it is higher than your Wisdom.
See also: Spell: Charisma
The statistic CHA. The Hidden Strength AA makes CHA over the cap increase the chance of landing spells.
From the in-game description: Charisma affects how much certain (the Charms vendor) merchants will charge you for their wares. Additionally, it plays a significant factor in how resistant foes will be to your spells. It also helps with some skills such as Intimidation.