The Weapon Triangle is the triangle that determines effectiveness between weapons. The triangle starts off with axes at the top. Going counter-clockwise, the axe is effective against lances, the bottom left of the triangle. Then, lances are effective against swords, the bottom right of the triangle. Finally, swords are effective against axes, back to the top of the triangle. It's a continuous loop between the three weapons, with one other weapon in the middle of the triangle: the Bow. Bows are excluded from the triangle, and are not strong or weak against any other weapon.
If a weapon is effective against another in battle, the stronger weapon gets +1 damage and +15 hit rate, and the weaker weapon gets -1 damage and -15 hit rate.
"Reaver" weapons can reverse the triangle, making swords effective against lances, lances effective against axes, and axes effective against swords. For example, if a Lancereaver was used against an Iron Axe, the Iron Axe would suffer penalties and the Lancereaver would gain bonuses. If two Reaver weapons are used, the triangle is "double reversed", and pretty much stays the same. However, if a Reaver weapon is used, the bonuses/penalties are doubled, the stronger weapon getting +2 damage and +30 hit rate, and the weaker weapon getting -2 damage and -30 hit rate.