Haki is a mysterious power that is found in every living being in the world. It is not that different from the normal senses. However, most people do not notice it or fail to awaken it. Broadly, there are two types of Haki common to everyone, given the proper training; however, there is a third type that only a certain group of "chosen ones" are said to possess. In simple terms, Haki is an ability to sense spiritual energy and overpower enemies.
Types of Haki
Haki is separated into three categories, or "colors": Kenbunshoku Haki, which grants users a sixth sense of the world around them and limited precognitive abilities; Busoshoku Haki, which allows the user to use their spirit as armor to defend against attacks or to make their own attacks more potent; and Haoshoku Haki, a rare type of Haki only one in a million can use which grants the user the ability to overpower the will of others. Most people who can use Haki tend to have a type they are better at and as a result focus on that type. However, a person can improve their abilities with all the types of Haki, it just requires more work.
Kenbunshoku Haki, is a form of Haki that allows the user to sense the presence of others, even if they are concealed from view or too far to see naturally. With enough skill, one can use this Haki to predict an opponent's moves shortly before they make them, thereby making the attack that much easier to evade. This prediction appears to the user as an image or brief "premonition" of what the opponent will do in the user's mind's eye, and the damage the user will take if the attack actually "hits". It appears that the more killing intent the enemy has, the easier they are to predict.
Busoshoku Haki allows the user to create a force similar to an invisible armor around themselves, this allows the user to defend against attacks that would otherwise cause them harm. Only significant physical force can overcome this defense. The invisible force can also be used, to enhance the attacks of the user greatly. This type of Haki also enables the user to grab unto the Logia bodies. They may also embed their weapons with this Haki to also allow to touch the Logia devil fruit users.
This type of Haki grants the user the ability to dominate the wills of others. The most common usage in the series so far is knocking those with weak wills unconscious. Whilst inexperienced users are restricted to merely overpowering the will of one individual or blindly knocking out weak-willed people around them, those with more expertise can pick out weak-willed individuals in a large group and knock them out without affecting those around them.