Characters throw shurikens by pressing the shuriken button. The travel speed, tracking, and number of projectiles is character specific.
Shurikens can be thrown in the air or on the ground. Most characters are only capable of throwing one set of shurikens on the ground at a time, with grounded shurikens having significant recovery lag that can only be parry cancelled.
Shurikens thrown by airborne characters prevent the character from taking any further action until they land.
Generally a character's air shuriken and ground shurikens behave exactly the same.
Shurikens generally have low hitstun and blockstun, and deal very little guard damage.
Characters in an Air Dash, Chakra Dash, or Air Chakra Dash are invulnerable to shurikens for the duration of the dash.
Shurikens can collide in mid-air with an opponent's set of shurikens. Whether all projectiles from both sets of shurikens disappear in the collision is based on relative location and shuriken type for both characters.
Loading one level of chakra and pressing the shuriken button will throw a set of chakra shurikens.
Loading two levels of chakra and pressing the shuriken button will throw regular shurikens and clear all loaded chakra from the character.
As a matter of fact, throwing shurikens clears all chakra from a character no matter what. Buffering a shuriken cancel in a combo and then loading chakra will throw regular shurikens while wiping the one level of chakra off the character instead of throwing chakra shurikens.
Shurikens thrown while ninja moving will extend the duration of the ninja move.
Shurikens are one of the three shuriken related actions capable of Shuriken Action Tilt Cancelling.
[List of Characters sorted by Shuriken type here]