The bokken is a practice sword used in the martial arts. If it's made of wood (usually white oak), it's a bokken. If it's made of bamboo, it's a shinai.

In addition to being used to practice kendo no kata, and open-handed techniques against bladed weapons, the bokken was used in Japan to fight duels with. Probably because it is inconvenient to die with a severed head over one's honor. Bokken would more often maim in the form of a broken limb, satisfying the dueling duty while allowing one to continue living.

Early kendo practitioners used bokken during training for sparring practice, as it was a marked improvement over using an iaito (dull swords). With the invention of the shinai, the bokken was no longer used for sparring, except by idiots.

I've been hit a number of times with a shinai on bare flesh (accidents at practice) and it hurts. It usually leaves a small bruise or welt. I have not been hit with a bokken, but it could easily fracture my limbs or cause a concussion. Bokken are not toys, they are weapons, and should be treated as such.