A stapler is a special mechanical device designed to force wire staples through wads of paper. It is then designed to bend the protruding ends over, which holds the wad of paper together. However, staplers usually fail in this simple task, and just fold the staple against the top sheet of the wad of paper. The shape of the staple is usually then some kind of groovy wavy wire type thing. After about 5 attempts at using the stapler, it is best to give up and use a paperclip (which is a far more versatile piece of bent wire).
Staplers come in many colours, shapes and sizes. Possibly the best staplers are the see-through electronic ones, which, although they are just plain gadgety, and fail to staple anything thicker than 2 sheets of paper, are still the envy of your work colleagues.
There are also devices called staple removers which are exceedingly useful for removing staples from wads of paper. Staple removers also look a little bit like Pacman with fangs.
There is also a European Staplers At Work Directive which requires that whenever a new employee takes over an old desk, he must be provided with lots of old loose staples in the back of one of the drawers, but may not be provided with a stapler.
Personally, I prefer Bulldog Clips to staples.