From Germaine :

A bokken (Bok = wood and Ken =  sword) is a wooden Japanese sword used for training. It is usually size and shape of a katana which is a Japanese sword or long sword. In use after the 1400s, the Katana is a curved, single-edged sword traditionally used by the samurai.
Bokken is divided in 3 parts :
-Tsuka
handle of the bokken
-Kissaki
tip of the bokken
-Tsuba
hand-guard that is sometimes slipped over the bokken
Historically, bokken were for the training of warriors.
 A bokken is used as an inexpensive and relatively safe substitute for a real sword, in training for several martial arts.
There are different types of bokken and they can be made of different woods as :
- Southeast Asian wood
- North American red oak
- Asian species of red oak
- Japanese white oak is the most common wood
- Hickory woods

Some studies done on wood for bokken use have found that Appalachian Hickory is a desirable wood for bokken use. It is strong and flexible, and does not lose quality over time.

However, no matter the quality or strength of a wood, no bokken is indestructible and all types of wood will break with sufficient stress.