I found this with a quick Google search:
Boars have prominent tusks (Sows have much smaller ones.) that never stop growing. The upper jaws have the stumpier "whetters"; the lower jaws the long, outwardly projecting "cutters". The whetters constantly shear the edge of the cutters (like a whetstone) making them razor sharp weapons that can fatally puncture an opponent or easily slice through a 3 inch tree root. The cutters are often worn or broken during fighting or rooting, but renew themselves with continuous growth and sharpening. When you see an agitated boar popping his jaws and foaming at the mouth he is actively honing his most lethal weapon.