All of these stats can be increased by wearing various items. The first five stats can also be permanently increased by putting points into them every time you level up and temporarily increased by buffs cast on yourself. Stamina can also be increased by character upgrades.
Attack vs. Defense
Attack and Defense determine your chance to hit your opponent in combat.
Generally: Attack > Defense = Hit Attack < Defense = Miss
More specifically, in each combat, a random percentage between -5% and +5% is applied to the attacker's attack (and another one to the defender's defense), meaning that it is recommended you have at least 10% more attack than your opponent's defense to be sure of success. Also, there is a 2% automatic chance that you will miss an attack regardless of your attack.
On the flip side, the more defense you have, the smaller the chance of your opponent hitting you. There is also the same 2% automatic chance that the opponent will hit you regardless of your defense.
Damage vs. Armor + HP
Damage, Armor, and HP determine the outcome of a round of combat.
Generally: Damage > Armor + HP = Kill Damage > Armor = Wound Damage < Armor = Miss / -1HP
When you hit your opponent, their armor absorbs part of the damage. Whatever damage is left gets taken from their HP. Because of this, you want to have more damage than your opponent's combined armor and HP or they will take multiple hits to kill.
Similarly, if you want to be safe from your opponent's hits, you need to have enough armor to absorb all their damage. Even then, if they manage to hit you, it is possible to lose 1 HP each hit.
Stamina determines how many actions you can do.
Stamina per Action Map Movement -> 1 Stamina Map Movement (diagonal) -> 2 Stamina Create Attack Group -> 20 Stamina Join Attack Group -> 5 Stamina Attack Creature (max 20) -> 1-20 Stamina Attack Creature (max 40) -> 1-40 Stamina Attack Creature (max 60) -> 1-60 Stamina Attack Player -> 10-100 Stamina
These three stats increase your Stamina, XP, and Gold every hour. Stamina will stop increasing as soon as it reaches your max stamina, but the other two will continue to increase regardless.