Strength, Agility, Guard, and Intelligence are all battle stats,these stats are critical to your success in a fight, without good stats even the best weapons and armor are useless.
Strength; the higher your strength, the more damage you do per hit.
Agility; is used to determine your hit rate in battle.
Guard; reduces damage taken when hit.
Intelligence; increases your chance to dodge other players hits.
Train your stats to the style that suits you, though don't forget that balance is important as well.
You can also increase your stats (and gain helpful items) with classes.