A balance board[1] is a device used as a circus skill, for recreation, balance training, athletic training, brain development, therapy, musical training and other kinds of personal development.
It is a lever similar to a see-saw that the user usually stands on, usually with the left and right foot at opposite ends of the board. The user's body must stay balanced enough to keep the board's edges from touching the ground and to keep from falling off the board.
A different challenge is presented by each of the five basic types of balance boards and their subtypes. Some of them can be attempted successfully by three-year-olds and elderly people, and some, because of their steepness and speed, are difficult and dangerous for professional athletes.
In their design, what differentiates the five types (and their subtypes) is how unstable each of them is, i.e., in how many and in which of the three dimensions of space each board turns and/or sways and how freely its fulcrum contacts the board and the ground.