Kunai, along with the shuriken, is one of the most common ninja tools. It is a black dagger designed for thrusting and stabbing, though it can still do some damage if thrown despite not being designed for it, and is about the length of one's hand.
Many shinobi attack by attaching an explosive tag to the blade. The kunai is usually thrown so that it lodges in an object near the target, and then the explosion from the attached tag destroys the target. This use of the weapon is known as an explosive kunai (起爆クナイ,kibaku kunai).
Kunai can also be kept on spring-loaded wires up the user's sleeve, as seen in Naruto Uzumaki's fight withSai when they first met. They can be used for close combat, like a normal knife. Empowered with chakra flow, kunai thrown into targets can cut through solid wood and even solid stone targets.
Shinobi appear to favour them so much due to both their sturdy and dependable nature, as well as their evident versatility.
(Kunai definition, Naruto style, curtesy of http://naruto.wikia.com/wiki/Kunai)