Just like with the shuriken and Ninjutsu, the exaggeration persistent in Ninja myths played large role in creating the current pop culture image of a kunai.
Currently, Kunai is commonly believed to be an ancient Japenese throwing knife. Designed as a weapon, larger than a shuriken offered increased accuracy, damage power and armour penetration when thrown, and could also be used in hand to hand combat more readily, and could even block sword blows. In addition, it could be used for climbing, as either a kind of grappling hook, or a piton.
The blade shaped like a squashed octahedron, a rod for the handle with a ring on the end, form an extremely simple yet effective design. It would be made of tempered steel, with sharpened edges, and polished to a shiny surface (For visual effect (metsubishi) or for aerodynamic abilities). Sometimes the clan symbol would be engraved on a side of the blade. The handle wrapped in fabric for better grip, sometimes with some strands hanging of the ring, to improve accuracy when thrown. (thus making it a kind of a throwing dart)
In Hatsumi Sensei's Grandmaster of the Togakure Ryu school of Ninjutsu collection there are examples of short kunai, long kunai, narrow bladed types, saw-toothed types, and wide bladed types. In some cases, the kunai and the shikoro are hard if not impossible to distinguish. A shikoro is a wide bladed saw with a dagger-type handle.