The aim of the game is to move the tiles so that they are in order from 1 to 15 with the blank at the lower right. Clicking on a tile will move it, provided it is adjacent to the blank
The puzzle was "invented" in USA by Noyes Palmer Chapman, a postmaster in Canastota, New York in 1878. The game became a craze in the U.S. in February 1880, Canada in March, Europe in April, but that craze had pretty much dissipated by July.