Properly termed
nunchaku, "nunchucks" were popularized by
Bruce Lee, as they were his
favorite weapon. They are very impressive in operation, and he often performed
intricate routines with "chucks" in his movies, most notably in a fight scene in
Enter the Dragon, where he kicked a large amount of ass with them and other
martial arts weapons.
As most basic martial arts weapons, the nunchaku is very simple, a consisting of two sticks roughly a foot to a foot and a half long joined with a short length of cord or light chain (cord is traditional, and won't hurt the user as easily as a chain will, although chain looks cooler.) This creates a short flail that can be used by holding either end.
In use, one holds one stick and whips the other towards the target, the hinge formed by the joining rope enabling the other stick to snap into the target with a high velocity due to the increased lever arm. An ambidextrous weapon, nunchaku can be passed rapidly between hands, between the legs, under the arms, and behind the head, in multiple attack/defense movements. It is common for a nunchaku artist to perform multiple movements simply to intimidate their opponent, as the whirling sticks create a formidable barrier to an attack.
One can also hold the weapon in a ready position by gripping one end in one hand and the other under the armpit, ready to lash out at bad guys. (It is customary to hold the other hand out in a martial arts pose while in this position, and call out "heeyaa"!)
You really have to see an old Bruce Lee movie, or at least watch someone of his caliber perform a routine with nunchaku. It is truly awesome to observe a master, for to watch an amateur is usually (sympathetically) painful, as they tend to whack themselves in sensitive places when they miss.