In the eastern martial arts, the bo is typically a wooden staff between one to two inches in diameter and between five and six feet in length. Length is determined by physical stature (height) and preference of the user. Also, the type of wood will be chosen according to the user's preferences in weight and hardness.
Historians have found the earliest records of such a weapon being used for self-defense residing in China where it was mostly used as a tool to carry grains or water by balancing the loads in buckets on either end of the staff and then putting the staff across the back.
In the martial arts, there is a brain-numbing plethora of techniques for using the bo. Many consider its primary uses to be defensive because of the staff's obvious parrying applications and tendency to render opponents unconscious instead of killing them. However, the bo does have as many offensive applications as it does defensive. A trained user can easily generate enough force to deal a skull-crushing blow, prevent the enemy from ever procreating, or penetrate a bad guy's head through the temple.
The bo is often the first weapon taught to martial artists because its techniques tend to apply to almost all weapon forms. Also, it doesn't take much training to become proficient with a staff. Mastery, however, requires years of dedicated training.