I figure I'll go ahead and start with the theorycrafting since I haven't seen anyone start one yet. Please note that these may be inaccurate and it may vary depending on classes, this entire testing was done on a level 27 Elin Archer.

- COMBAT -
Attack - Modifies the damage of an ability.
Defense - Decrease the amount of damage taken from hits.
Impact - Increase your chance to knockdown enemies. (Formerly known as "Knockdown")
Balance - Reduce the chance of you getting knocked down. (Formerly known as "Resist to Knockdown")
Crit Rate - Increase the chance to gain a critical strike with an ability. (Formerly known as "Crit Chance")
Crit Power - Determines how much a critical hit is multiplied by.


- STATS -
Power- Increase your "Attack" attribute. (Formerly known as "Strength")
Endurance - Increase "Defense" attribute.
Impact Factor - Increase your "Impact" attribute. (Formerly known as "Damage")
Balance Factor - Increase "Balance" attribute. (Formerly known as "Balance")
Attack Speed - Increase the speed of your ability execution and charge abilities.
Movement - Increases your out-of-combat run speed while moving.


- RESISTANCE -
Section removed, its a pretty straight forward description on the in-game tab now


- GEAR STATS CONVERSION -
Archer:
1 Attack Modifier = 5 Attack
1 Defense Modifier = 1.25 Defense
1 Impact Modifier = 0.5 Impact
1 Balance Modifier = ~0.29 Balance

Berserker:
1 Attack Modifier = 5.25 Attack
1 Defense Modifier = ?
1 Impact Modifier = ?
1 Balance Modifier = ?

Lancer:
1 Attack Modifier = ?
1 Defense Modifier = ?
1 Impact Modifier = ?
1 Balance Modifier = ?

Mystic:
1 Attack Modifier = ?
1 Defense Modifier = ?
1 Impact Modifier = ?
1 Balance Modifier = ?

Priest:
1 Attack Modifier = ?
1 Defense Modifier = ?
1 Impact Modifier = ?
1 Balance Modifier = ?

Sorcerer:
1 Attack Modifier = ?
1 Defense Modifier = ?
1 Impact Modifier = ?
1 Balance Modifier = ?

Slayer:
1 Attack Modifier = ?
1 Defense Modifier = ?
1 Impact Modifier = ?
1 Balance Modifier = ?

Warrior:
1 Attack Modifier = ?
1 Defense Modifier = ?
1 Impact Modifier = ?
1 Balance Modifier = ?

Note: These stats still remains unknown weather its calculated as percentage or rating.


-- Unanswered Question / To Do List --
Attack --> how does it scale to the actual damage from an ability?
Defense --> how much damage does it actually mitigated?

There's still a lot of theorycrafting left unknown, feel free to contribute whatever you guys can find!

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What race/level/class/equipment did you use to get those multipliers for the gear stats conversion? They're not the same as what I have.

Its tested at a lvl 27 Archer, I tested with all equipment, from white to gold and lvl 1 items to lvl 27 items, the numbers are direct conversion at least for the Archer.

Feel free to share your stuff thus what you have done so far.

When is TERA coming out?

I give this only because it's not really useful. ;)

For a berserker, attack function seems to be

( (unknown) + (0.03 * STR) ) * ( (weapon attack modifier) + (base attack) )

I believe game rounds down.

I've only just began to test out all the functions. I dunno if I'll bother to finish them because they don't really represent a lot of practical value. Unknown part still reflects portion of attack multiplier that I dunno if it's just constant or affected by other things. There's also additional multipliers from crystals and stuff of course.

Good read overall can't wait till you add more info regarding crystals.

every skill has a atk power that is added to your base atk for calculating damage like for archer the normal shot at lvl 1 has a power of 9 so 9+base atk= damage done and on attcks that are charged the listed atk power is for fully charged atk and with like your multi shot its listing are 10-7-7-7-7-7-28 so your first and last hits are the strongest with the middle ones having lower damage so it makes it to your advantage to complete the set but for thous who need to run in a hurry at least the first shot is ok damage and the base atk goes up with each new version of the skill