Shuriken are a type of weapon commonly associated with Ninja. Typically seen as a thin metal star and thrown across distances to pierce the enemy, it is actually this and more.
The word shuriken itself has a complicated meaning. It is broken down to 'shu'(hand)'ri'(hidden)'ken'(blade), and thus was considered to be any bladed hand-weapon which could be concealed. At some point a variation of the character 'ri' may have been used by accident which led to the translation of 'hand release blade', and thus more specifically in reference to a blade which is thrown. The reality is, it is more along the lines of both. Typically the perfect shuriken is one which can be concealed and is usually thrown, but not always. This includes stars, knives, darts, and others. There are two particular basic types of shuriken: Shaken - The flat, thin, multi-pointed version typically thought of. Broken down into the two subtypes of hira shuriken which are star-shaped of various numbers of points, and senban shuriken which are lozenge-shaped blades. Bo Shuriken - Typically long and thin. There are three sub-types. Hari gata which are cylindrical and needle-shaped. Kugi gata which have a square base and are nail-shaped. Or tanto gata which are flatter and wider and are knife-shaped.
Shuriken vary between different schools (or ryu-ha), and so do the methods of use. They are all able to be thrown, in various ways depending on the school, but are also typically used for stabbing at enemies in close combat as well. In both cases, they are never meant to be used as a primary method of killing an enemy, as they are unable to cause that much damage (unless a precise hit is made in the throat). Typically they were moreso used as a distraction or a way of slightly crippling the opponent by hitting weak points such as the eyes, and causing vicious sharp pains.
 Typical lozenge-shaped senban shuriken.
 Various hari gata needle-shaped bo shuriken from the Shirai Ryu.
 Various shaken of both types, mostly hira-shuriken, from the personal collection of Dr Masaaki Hatsumi, 34th and current Grandmaster of Togakure Ryu Ninjutsu.
 The correct method of holding a thin hari gata bo shuriken in preparation to throw.