*but it still looks like a circle to me
Because this is Fire Emblem everything has a strength and a weakness. Weapons and magic do not escape this rule.
With weapons it's rather simple, all weapons fall into three main categories; Sword, Lance, and Axe. Knives fall under Sword, and Staffs do not appear on this triangle... circle... thing.
Sword is strong against axe and weak against lance.
Lance is strong against sword and weak against axe.
Axe is strong against lance and weak against sword.
With magic it is the same way, where all magic is put into three categories with the exception of healing magic which is not present.
Anima magic is strong against light magic and weak against dark magic.
Light magic is strong against dark magic and weak against anima magic.
Dark magic is strong against anima magic and weak against light magic.
On a special note, Anima magic means all the elemental magic: Fire, Lightning, Wind, and Ice.